I'll rephrase this a little.
The "define" allows to to assign ADDITIONAL names to a skill. Think of it as a "nickname".

EG
#define health skill5
#define damage skill5
From here on in, if you change/read my.health you are changing/reading my.skill5,
AND if you change/read my.damage you are ALSO changing/reading my.skill5.
The 'define's are not linked to any specific entity.
So if you only use my.health with a player or enemy entity, then skill5 is its health.
And if you only use my.damage with bullet-type entities, then skill5 is how much damage it does.
ALSO you can CONTINUE using "skill5" as its name if you want, if the entity is not a player, enemy, or bullet.