Description of entity type properties
I had to re-install to get it to work, AFAIK.
The data_table is initially empty, but gets filled up as soon as you start creating a few of your custom entities. But the table itself should be there.

What exactly the data_table is for, I'm clueless as well, as pretty much everything worked without declaring it.

I couldn't however get the Entity Autocomplete ( see here for more info/example ) form element to work without declaring the data_table.
As soon as I added the annotation, the autocomplete element worked by declaring #target_type =&gt; 'my_custom_entity'