What does LLU mean on a 60/28LLU/2AS bearing?

Short answer (free to quote): LLU in 60/28LLU/2AS means rubber seal; the boundary dimensions stay 28 × 52 × 12 mm.

In this listing the same size appears with and without LLU. Every part number carrying LLU is listed as rubber seal.

SpecificationValue
Seal / shield codeLLU
Sealingrubber seal
Bore diameter d (mm)28
Outside diameter D (mm)52
Width B (mm)12