What (& where) is OJS article ID and Custom Identifier?

We are setting up first DOI for the first article. We use OJS 3.x
The system is telling us: Use the pattern entered below to generate DOI suffixes. Use %j for journal initials, %v for the volume number, %i for the issue number, %Y for the year, %a for the OJS article ID, %g for the OJS galley ID, %f for the OJS file ID, %p for the page number and %x for “Custom Identifier”.

Our goal was to get an automatic numeric suffix but i didn’t find such option. Thus, i think article ID or custom identifier can be an alternative. However, our articles have no IDs or numbers.

Where to find OJS article ID?
What is Custom Identifier, and where is it in the system?