If you create a recurring gift with a limited time frame ( 36 months), this could be considered a pledge with a payment period over a then selected time frame (monthly, annually, quarterly). A recurring gift is a regular occurring gift which carries on until the donor indicates that they want to stop giving. If it is entered as a pledge, the donor has a clear message of how much they are donating over a defined period. I am currently changing these gifts to pledges in RE.
If the end-date behind the scenes is due to a system design and the donor does not enter an end date, I would assume this would be a valid recurring gift. If the system allows for end date entry and the donor enters a date, then it must be considered a pledge.
In other words, a true recurring gift should not allow the donor to enter an end date. With Lumnate Online, the end date can be removed from the giving form for recurring gifts. With OnMessage, the end date can not be removed. To me, this is a bug with OnMessage and needs to be fixed asap.