Column Insertable property
If Column Insertable is set to No for a column:
On inserting a new record, this column will have read-only appearance: the user cannot enter data in the field. Nor is it possible to enter data in another way, for instance using a set-values() action, or by duplicating a record.
If the record is stored, you are no longer inserting, but updating: you can now enter data in columns that were set non-insertable.
The exact behavior of the column depends on the combination of settings for Insertable, Updatable and Input Allowed.