A sequence is a data base object that are used to generate unique integers which are used for creating [ID] values.  Each table will have an associated sequence and a [data base function|DATA BASE FUNCTION] to determine the next sequence value.  The sequences generate a unique sequential number, but by virtue of the way that the references for next sequence value are made, there is no guarantee (nor a requirement) for consecutive values.

The [ID] fields are system-managed and are completely internal to the system.

This is unrelated to the application Code Sequences, managed by the [IMCS] function.