Metaforce User(s) are any character that have Meta Points, and can use Meta-Powers. Player characters are almost always Metaforce Users by this definition as they start with some amount of MP.
This title and status is not known to characters that have it, and simply describes how their fate is often shaped by the players who control and portray them in the Metaforce system. Narratively this shouldn't be mentioned unless it is an official Metaforce story or campaign.
The official description of what the Metaforce is will be written in own article.
Protections
Metaforce Users are special in that they are also protected from negative influences from other Metaforce users. Whereas a Metaforce user can use their M-Powers to influence the results and values of non-MForce users, MForce-users can't have their result or values affected by others.
A Metaforce user could use Create a Meta-Die to reduce the ToHit of a random enemy's attack roll, but they couldn't do so if that enemy was a Metaforce user. They would have to manipulate some other value instead.
Gaining/Losing Status
It is possible to become or lose the ability to become a Metaforce User. This all boils down to whether or not they have Meta-Points, specifically the capacity to hold any amount of Meta-points (Max MP).
If a character has their Max MP reduced to 0, they are no longer a Metaforce user. Any M-Powers they have equipped will be disabled and no longer benefit from their passive or active effects. They will become vulnerable to other Metaforce users applying their powers upon them. They will not be able to spend Experience in order to raise their MaxMP, although they still will gain Max MP from level ups and other sources.
Inversely, a character who gains Max MP will gain the ability to equip M-Powers and have any disabled ones return to working order. They gain protection from hostile interference from M-Power usage.