Custom fields and extensibility

Adding a field the platform does not ship, without a release and without touching the core tables.

A field your business needs and the platform does not ship is added as a custom field. It is defined per company, attached to a resource, given a type and a label, and it then appears on that resource’s form, its list columns, its filters, its import mapping and its export.

Custom fields live outside the core tables, so adding one does not alter the schema every company shares.
They are ordinary fields once defined — filterable, sortable, importable and exportable, not a note field with a label on it.
They are per company, so one company’s additions are invisible to another.

Before adding one, check whether a classification list does the job. If what you want is a new value in an existing dropdown, that is configuration, not extension.