The arrays to connect souls to armour are really different for the all three suits we know. I'm sure I've read a post about it before, but I couldn't word a search that would make it pop up, so here's at least a small comparison.
Al's seal (picture from Chapter 9):
Al's is really simple. A circle because that's alchemy - the equivalent exchange/orouboros symbol of cycles, of wholeness in the process of change, etc. I think the starry kind of bit indicates a container, and the swirl is the soul being held. I'm making this up, but look at it.
Now let's look at the two Slicer seals.
Slicer's seal (helmet) (picture from chapter 11):
Slicer's seal (body) (picture from chapter 12):
There's the circle, and the star (fewer points this time - a Star of David).
So here's where my brain goes "is this about the Jewish connection to alchemy?" since I know there are connections, particularly the Sephirothic Tree that appears on Ed's door to Truth, "or since circles are special, do triangles mean anything special in alchemy?" and let me tell you: yes.
(First, this search led me to find that combined with the circle, this is more likely to be a reference back to the Seal of Solomon, from which I have now learned the Star of David was derived. The Star of David is more about the Jewish identity and Judaism, whereas the Seal of Solomon is connected to Jewish, Islamic and occultic and alchemic mysticism.)
To paraphrase the relevant parts of this site, a downward pointing triangle with a line through it (which these conjoined triangles give each other) symbolises Earth, considered a symbol for physical sensations/movements; while an upward pointing triangle with a line through it symbolises its opposite, Air, which is considered a symbol for the Holy Spirit and life-giving forces/sources of life such as breath (this source says it's associated with blood).
Slicer's seal is basically "give physicality and life"!
What then does that mean for Al's seal?
With those extra lines, what could the starry bit on Al's seal mean? It looks like Slicer's, plus more. I couldn't find anything on triangles pointing sideways instead of up or down.
If there is an intention behind it - which there so often is with Arakawa's little details, but I don't want to assume she always has to be meaning something when she was publishing chapters monthly, as she wrote them, and didn't get the chance to go back and edit as the story progressed. If, I say, there is an intention behind it, I wonder if it's like an inclusion of the two other triangles. Representing the other two elements (fire and water). They don't have a line through them, but consider this:
Fire represents the 'fiery' emotions (love, passion, compassion, hate, anger). It is considered masculine/male, and symbolises rising energy, aspirations to reach the divine above us.
Helloooo Father, and Roy Mustang.
Water represents intuition. Considered female/feminine. Combines with the Fire triangle to make the Seal of Solomon, which, if significant to alchemy, could be like Ed going "double powerful" by having the fire/water one plus the earth/air one.
But look at that. While Slicer's seals, designed by the military alchemists who were happy experimenting on people, are concerned with connecting motion and life, Al's has that plus Ed's care for his emotions and intuition (feelings and thoughts). Isn't that sweet? ;_; I wonder if it makes any difference.
Maybe there's more than one way to get a job done.
The only difference I can think of is how stable/unstable Al's connection is.
So in addition to the circle (alchemy) and the triangles (tethering, restoring), Slicer has the squiggles in the middle (soul?) and those lines on the outside.
I haven't uncovered any symbols that shed light on those, but it puts me in mind of in later chapters, Roy having his legs pinned by the unsuccessful Fuhrer candidates and his hands pinned by swords. Like swords pointing at the soul in the sigil, warning it to stay put.
Barry's seal (picture from chapter 39):
Al gets this quickly formulated love-letter to alchemy begging for human life in all its beauty, Slicer gets a command to be functional, and Barry gets ... a whole new equation.
And we've just seen the mural on the wall under Lab 3, with the sun and the moon and all the alchemical theory to be hinted at later, so you know by now (30 chapters after the first seal) that Arakawa has an idea of what she's doing.
Circle - that's familiar. But with a second circle inside! New, chic, beautiful. Between the two circles there's some fancy writing. Too small to be read (I bet she was getting tired of coming up with new alchemical symbolism to add but it seemed cool, or it was meant to reflect the precision of the alchemist making it).
Something in the middle - it's a bit different. An arrow crossing a boundary, within a squashed rectangle.
Something on the outside - the flourishes on the leftmost side of the extruding bits look not like the head of an arrow, but similar in signifying a direction. Maybe following the same direction the inner arrow is, like to say you poke his soul out from the body and into something else.
And I think this may be a significant answer to why Barry's array is so different:
As you can see, when Barry's body (whoever is in there, with very little of their mental faculties remaining to them) destroys the array, it kills the both of them.
The one array is tethering both of them.
Its considerable complexity is something Ed would never reach for - he's not looking to kick a soul out of their body - and we don't hear anything about the Slicer Brothers' bodies, so maybe they were disposed of in a regular way - or simply a different one.
Barry tells us a soul can reject a body - or a body a soul - and that's why this man is deteriorating so much. His body is rotting and the soul is incoherent, while the metal containers have kept the armoured souls more stable. The array on Barry's armour is too distant to maintain the body's life, if it is even designed to do such a thing.
All the arrays are different. Barry's serves a significantly different purpose, with the additional soul to pin down. Al's lacks the external flourishes that could act as tethers. Slicer's is duplicated for the brothers, except for the individualised soul mark in the centre - showing us that they don't all have to be unique in design.
And a lot of the stuff in there is researched and intentional, instead of just slapped together.
The attention to detail even in the variations of symbols meant for the same purpose gives so much to chew on. This is why FMA is such an enduring series.
