The character is familiar with the use of crossbows. Unless a character is familiar with a particular crossbow, some practice is required to adjust to that weapon's aim, range, and shot strength. Until the character has fired 10 shots with a specific crossbow to get a feel for it, she suffers a -20 AV to all shots fired.
Crossbows are now treated as a weapon proficiency tied to the Aim skill. The cost of this proficiency is 30 Experience or 6 Skill Points.