How To Create New SLA Event Class And Journal Category in Subledger Accounting?

In Release 12, with the introduction of SLA (Subledger Accounting), the system uses only the seeded journal categories. Custom journal categories used in previous releases can no longer be linked in the Book Controls form.

The Journal Categories tab is missing. And, in the Accounting Event Class Options form
Setup > Financials >Subledger Accounting > Accounting Methods Builder > Events,
the seeded event class Adjustments cannot be edited nor can a new event class be inserted for Assets.

The only possible option is to update the seeded Adjustment journal category
GL: Setup > Journal > Categories
and change the category for category key Adjustment to FA Adjustment. Note, however, that this is the seeded journal category which means that it can be updated/changed by a patch or upgrade