Alternate text for table and image accessibility - textobject, alt, summary?

Apologies in advance if this is the wrong list. I've had conflicting advice about what to post to docbook vs. docbook-apps.

I'm brand new to DocBook, and am reviewing the current source files for the doc set at my new job.

Is there a recommendation for providing alternate text for accessibility?

The current source files have textobject for images and nothing for tables.

Is the alt element better than textobject?

For tables, should I use alt or summary?

Thanks,

Janice