Assassins must choose one of the attack modes: regular attack, backstab, or assassination. Though a failed assassination still qualifies for regular weapon damage, it does not permit a backstab.1)
During gameplay, assassins receive combat XP for killing assassination targets and XP for any fee, regardless of whether the roll on the assassination matrix succeeds. However, XP for the assassination itself (not combat XP) is only awarded if that roll succeeds. Assassins receive assassination XP for any assassination, including assassinations during regular adventuring (i.e. when not “on a job”). Unless the DM declares otherwise, these are “simple” difficulty (½ XP modifier).
When handling abstracted assassination missions during downtime (when not adventuring), if the assassination is abstracted down to a single die roll and there is no chance of harm coming to the character, no combat XP should be awarded for the assassination, only GP XP and assassination XP. However, if a risky system such as Downtime AssassinationEx is used, combat XP should be awarded.
Non-assassins who score a kill by means of the assassination table (such as by attacking a helpless target) do not earn any assassination XP. The regular XP value of the monster is still earned by the party, however.