An abbreviation is a sequence of alphanumeric characters that you type to reach expansions.
Instant Text keeps track of the last abbreviation you type, in order to suggest potential expansions in the Expansion Table.
Note that there is a subtle distinction between short forms and abbreviations.
But there is more to it, the short form defines a complete scope of possible abbreviations to reach an expansion. And don't get it wrong, long short forms do not not imply longer abbreviations. They just enhance a larger scope of possible abbreviations.