letters after airline callsign

I'm listening to the LiveATC.net feed of Norcal (SFO) Approach,
since the weather is 'fun' today[1], and a few of the airliners are
using letters after their standard callsigns. Two I've heard are
United 893 Lima and United 885 Kilo. They show up on FlightAware
as UAL893L and UAL885K, and UAL893 and UAL885 also show up, but as
different aircraft types and different routes.

Is this United's infamous 'change of gauge' 'direct' flight trick?
According to FlightAware, UAL893L is a 757 flying KJFK-KSFO and
UAL893 is a 777 flying KSFO-RKSI (Seoul, Korea).

Does the letter have any special meaning beyond that it appears to
be another flight using the same flight number but a different
aircraft? UAL885K and UAL885L both have been used according to
FlightAware, in addition to UAL885.

John
[1] Lots of missed approaches due to windshear. Diversions due to
RVR 19R OTS and crappy vis.
--
John Clear - http://www.clear-prop.org/