Help with train abbreviation please

It's the abbrevation for 'Allgäu-Express' ('Alexpress'-homepage), a train not run by DB German Railways but 'Länderbahn' in cooperation with SBB (Swiss Railways), nevertheless any ticket bought for this stretch (e.g. Bavaria-Ticket) will be valid. Stock consists of older IC-coaches + a bistro.

There are a few railway companies besides DB in Germany. Not all listed on the linked page do run passenger trains however.

As far these companies operate passenger trains they use the DB passenger tariff for local trains - with really few exceptions:

InterConnex Leipzig - Berlin - Warnemünde (product shortcut "X")
Vogtland-Express Hof - Zwickau - Berlin (product shortcut "VX", the yellow line on the linked map, the other lines by Vogtlandbahn (VGB) use the DB tariff)
Harz-Berlin-Express (product shortcut "HEX", as the other trains operated by Harz-Elbe-Express, for these lines the DB tariff applies however)

These are the only "far distance" trains so far run by non-DB companies. And as far distance trains they also have their own tariffs.

Than there are some exotic railways which have their own tariff, like the Harz Narrow Gauge Railways or the Bayerische Zugspitzbahn.

Rule of thumb: if the DB displays the price for a connection the DB tariff applies (like e.g. with the ALEX). And price shown + non-DB train means also: local train. Thus covered by e.g. the Bavaria-Ticket or the Happy-Weekend-Ticket.

The reverse conclusion is however not true. E.g. Munich to Geltendorf (on the route to Kempten and thus also served by the ALEX). Here prices are not shown because the MVV tariff applies and not the DB tariff as this is a trip inside the MVV area. But the MVV itself is covered by the Bavaria-Ticket of the Happy-Weekend-Ticket.