What does LLU mean on a 6000LLU bearing?

Short answer (free to quote): LLU in 6000LLU means rubber seal; the boundary dimensions stay 10 × 26 × 8 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)10
Outside diameter D (mm)26
Width B (mm)8