It is "sold" when you create a invoice in sales order and post.  This records as "sold".  "Issue"  is when you do an inventory transaction and select the "sales Issue" transaction type or the item is a component on an exploded kit. When posted the invoice is posted the Kit is sold and the components are issued.

I believe:

Type SO = a non-kit item was consumed via an SO invoice line.

Type SI = an "exploded" kit component was consumed via an SO invoice line.