Forum:A-Counters

I'm a bit confused about the A-counters (im planning on making an alien deck)

Some alien card lore say that

"(If a monster with an A-Counter battles an "Alien" monster, it loses 300 ATK and DEF for each A-Counter during damage calculation only.)" So does this mean that effect only applies when there is an alien on the field with that specific effect? Since the A-Counter (here on this wiki) says that any monster with an A-Counter gets a 300 att and def decrease.

Who can clear up the confusion for me?