What is the yellow marking underneath Delta A220 doors? I've recently noticed a yellow marking underneath the left door of Delta A220s. It's a yellow rectangle with a black T on it and it is exclusive to Delta and no other airlines on the A220. Any ideas what it's for exactly?

Thanks!
This is almost certainly an alignment marking for the passenger boarding bridge (jetway). You can find the same markings on other Delta aircraft as well:


(image sources: A320, B737, B767)

The dead giveaway is that these markers are found on the second door on the long haul jets, which are typically boarded through this door instead of the first one:


(image sources: A330, B777)

The jetways used by Delta will have a similar marking on them which aids in properly aligning them with the aircraft door such that the door can still be opened and passengers can safely board and disembark.

See also: What are these markings under the passenger door?