UN-xx is Shimano's Bottom Bracket generic code. Higher numeric numbers are of superior material or design or construction. I too have UN-55 on at least one of my bikes. I bought it because the UN-2x I wanted, which was the previous best value almost-generic Shimano BB had gone out of stock everywhere simultaneously, all of a sudden. Without any announcement that anyone here saw (we discussed or at least mentioned it about ten years ago), Shimano had made the UN-55 their new quality/value standard. COVID-19 upsets aside, I expect that soon a UN-xx where the the xx is definitely a number higher than 55 will be the Shimano price/performance leader at everyone's LBS and from all the mail-order houses.
Shimano UN-71 Bottom Bracket, photo ex Ebay
Trivia: Through all this, the best (highest numerical coding) Shimano square taper BB has been the near-mythical UN-71. I've never met anyone who has seen one in the flesh though they have been offered for sale; here one sold for just short of USD140 delivered:
https://www.ebay.com/itm/New-Old-Stock-Shimano-BB-UN71-Bottom-Bracket-70x115-mm-Made-in-Japan-/272192192462 The question arises, When Shimano runs out of numbers below -71, do they give us the -71 for a popular price? My bet is they'll renumber the -71 as the UN-101 to give themselves breathing room.