All multi-spec identities are valid!
There are many identity labels that people who are attracted to more than one gender (multi-spec people) can choose from. They often have overlapping definitions and this is perfectly fine because all multi-spec people have shared experiences that are unique to being attracted to more than one gender.
It is entirely down to an individual to choose the label that they feel fits them best and there is no wrong way to be multi-spec.
Many people identify as bisexual, this is defined as the attracted to two or more genders. Some bi people are attracted to men and women, and some are attracted to all genders and there are a multitude of experiences in between.
I call myself bisexual because I acknowledge that I have in myself the potential to be attracted – romantically and/or sexually – to people of more than one sex and/or gender, not necessarily at the same time, not necessarily in the same way, and not necessarily to the same degree.
This definition states there is no wrong way to be bi, you might be biromantic and asexual, bisexual and aromantic, 75% attracted to women and 25% attracted to other genders, or feel gay one day and straight the next, all of these are legitimate experiences for bi people.
Another common multi-spec label is pansexual, this is a label that is more specific than bisexuality because it clearly states you are attracted to people of all genders. Some people will ID as bi and pan because they are attracted to all genders and they feel some connection to both labels.
Pansexual people can also have preferences for different genders, and may also be ace-spec or aro-spec, again there is no wrong way to be pan, you don’t have to date or sleep with people of every gender in order to prove your sexuality - and you certainly don’t have to be attracted to everybody.
There are less common labels like polysexual and omnisexual, polysexual means you are attracted to multiple genders, and people often use the label to make it clear there are some genders or a gender they aren’t attracted to so pansexual wouldn’t be a suitable label for them. Onmisexual means the same as pansexual but is a label with a different history and community.
There are some multi-spec labels that are specific to different cultures or gender identities, and these should only be used by the people who are part of those cultures or identify with those genders.
At the end of the day, the definitions are guidelines to help multi-spec people decide which label they feel fits them best, and there is no harm in trying different ones over time if you find that one feels better, or to talk to people from the different communities to find out about the history of that particular label, or simply check out the pride flags and see which one you prefer!
All multi-spec people share the experiences and oppression that comes with being attracted to more than one gender, and all multi-spec people are welcome in the LGBTQIA+ community, whichever label they chose to describe their own unique experiences.