Does anyone know what (other than random things you CANNOT do) between an eTicket and eCredit?

I don't recall ever getting eTicket credits until recently. And it seems Delta is going that way now? Even though the web page (when you cancel) AND email both state: eCredit (but you get issued an eTicket).

AFAIK, the only differences are that you cannot use an eTicket in conjunction with a Delta Amex Companion Cert, but there may be other differences I'm unaware of?

The "conversion" process is (IMO) similar to how UA handles their FTC/ETC, sorta kinda, only that the expiration date extending seems to require a double/triple step/hoop.

But the UA forum has a nice thread about each type and what/how they work, whereas I couldn't seem to find anything on the Delta forum about eTicket vs. eCredit, hence my asking.