I think with Miss Cain it’s usually Cass or Cassandra, most likely to not confuse people with Cassie Sandsmark/Wonder Girl.
@Jay_Kay he nailed it on the head. Cassandra is Cass. Cassie is for wonder girl.
Bruce and Barbara have called her Cassie a couple times but most of the time she’s called cass
I don’t think “Cassie” is technically incorrect, but yeah, “Cass” appears more frequently and is generally favored to distinguish from Wonder Girl.
While Cass has been called “Cassie” at times in the comics, this itself is often a mistake. Just as Barbara is usually “Babs” and not “Barb,” Cassandra Cain is “Cass.”
The reason this distinction is important is because it provides an easy way to distinguish Batgirl and Wonder Girl by their first names alone.