ACL and PIX.

Is it true that the behavior of an ACL differs by a bit between PIX a routers.

I can seen on my PIX 6.3(4) that I need to apply the ACL with the command "access-group" for the ACL to take effect.
In this way I can modify the ACL the way I like without taking care of the traffic flowing through the device.
I always modified ACLs using PDM but now I need to use CLI.

Could you confirm that?

Alex.
.