A lyric that originally confused me a bit in Provider was, "your outer shell; your secret insider." At first, it seemed a bit of a backwards juxtaposition, one that didn't match up as well as others in their discography.
My first thought was that it was referring to being someone's 'protector' of sorts. Existing as a hardened exoskeleton, yet simultaneously managing to settle under the skin of the very person they're shielding. Barely even there, but making a home where it's well deserved.
Then, I remembered the chemistry motif. The "outer shell," typically refers to the last 'shell' around an atom, otherwise known as a valance shell. Basically, electrons surround a nucleus in varying levels (first shell holds two, second holds eight, the third eighteen etc.), and, the number of electrons on the furthest shell decide its chemical properties (how reactive it'll be, is it flammable etc.), and, if it'll be "acid or alkaline."
Neat little possible callback, I thought.
Furthermore, if an atom has an incomplete outer shell, it'll be unstable, but through a covalent bond (in non-metals), can be stabilized if another molecule shares it's electrons.
In short, they become equal; on stable ground with one another. To be someone's "outer shell," means to have a chemical, balanced bond with them; to keep them steady.
I think that really fits, considering the title of the song.