When is character evidence admissible in criminal cases?

How do California's rules differ from the Federal rules when it comes to introducing character evidence in a case involving sexual assault or child molestation? How is the process of offering character evidence generally handled in California? How is the process of offering character evidence generally handled under the Federal rules? If the defendant offers evidence of a victim's bad character, what can the prosecution offer to rebut? In California, if the defendant offers evidence of a victim's violent character, what can the prosecution offer to rebut? In California, when can prior acts of domestic violence or elder abuse be admissible? In cases involving sexual assault, when may a defendant introduce specific act evidence? What are habit and custom evidence? What is character evidence? When assessing character evidence, what are non-character purposes? When is character evidence admissible in civil cases? When is character evidence admissible in homicide cases involving self defense? What kinds are admissible? When is evidence of prior sexual assault or child molestation admissible?