Objects can be in multiple families. If there are any variables that apply to both enemies and friendlies - you can define them on AllEntities family, which contains all objects.
You will be able to do damage to both with a single line of code:
AllEntities subtract 1 from HP
But these variables will not be available in Enemies and Friendlies families. So you can do this:
Skeleton subtract 1 from HP
But not this:
Enemies subtract 1 from HP