When I'm flying to destinations outside Europe, it's usually a lot cheaper to book separate tickets to get me from TLL to some European city with decent connections and price competition, and then a separate ticket from that point onwards. This also gives me a wide range of *A choices as an EBG.

Most recently (December/Jan) I booked one itinerary OV TLL-FRA-TLL and then another UA FRA-IAD-FRA. These are separate itineraries/PNRs.

Upon checking in at TLL, I was told by OV check-in staff they couldn't tag my baggage all the way to IAD, so I retrieved it in FRA and went to check in the usual way. I had ample time for this, so no problems.

On the return, upon checking in at IAD, I was able to check my bag all the way to TLL. This was done by UA check-in staff.

Apparently this is all governed by whether the airlines involved have interlining agreements for baggage.

What confuses me is that there must be such an agreement in place between OV and UA if I was able to do it from IAD, but why couln't they do it from TLL? Do they simply not know how to do it, or they weren't allowed to do it (perhaps as I was flying to the US?)?

Knowing the details about things like this makes it easier for me to plan more trips where I use separate itineraries, in terms of how much transfer time I need between flights. If I have to retrieve baggage and check in again, I need to add 2 hours to the transfer time just to be safe.