Called Shot only works on creatures with distinct features, which means you can't make called shots on big tentacly blobs. (Whether a creature has distinct features or not is up to the GM).
Making a called shot adds the following modifier to the target's Armor Class:
|Tiny||No Called Shot|
|Small||+6 to Ac|
|Medium||+4 to Ac|
|Large||+4 to Ac|
|Huge||+2 to Ac|
Making a successful called shot to the head is an automatic critical (if a critical hit can be done to the target).
Making a successful called shot to a limb will reduce speed by 20% and/or attacks according to the limb being hit. Arms take a -2 to attack and any skill in which use of the arms are required, such as Climbing.