A self-unit is one that represents your whole side in some way. For instance, in a dungeon exploration game, your "side" might consist of an adventurer (you), your possessions, your followers, and perhaps more. In such a case, if the adventurer dies or is captured, then the game should be over, at least for you.
Usually the self-unit will be set up by the game design, and all you
have to do is to be aware that losing the self-unit permanently ends
your participation in the game. Some games might have "self-unit
resurrection" (self-resurrects
) which just means that if another
type of unit is available when the current self-unit dies, then that
another unit becomes your new self-unit. For instance, admirals would
leave their sinking flagship and board another ship, thus "transferring
the flag". (Admirals presumably being more valuable than captains, who
are supposed go down with their ships!) Some games may also allow you
to change self-units manually (self-changeable
). In any, the
game will define which types may be self-units (can-be-self
).