What does the U mean in UTM=(32, 'U')?

UTM 32U is one tile of the UTM 32N zone. This image from Wikipedia (here the link to the following image) should explain it clearly:

The 'U' in this UTM notation refers to a specific lattitude band I believe- see 'Latittude bands' here - https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system