Where a modus operandi (MO) concerns the practical components of a crime which can also be unique to one suspect, signature aspects fulfill a psychological need and, unlike the MO, do not often change.
Two examples cited in Crime Classification Manual by John Douglas are a bank robber from Michigan who required tellers to undress during the robbery so he could photograph them, and a rape case where the perpetrator forced the husband to return home and be humiliated by the event.
These characteristics move beyond modus operandi, because they fulfill a psychological need rather than a need of practical execution of the crime.
[1] The 1898 Gatton murders also exhibited signature aspects.
[2] Ted Bundy also used a complex series of signature behaviours.