@glhcarl
Thank you very much for your answer.
Still it remains unclear for me whether the mentioned ISB is mandatory for the AVRO RJ or not.
In line #1 of the document "BAe 146 SERIES/AVRO 146‑RJ SERIES AIRCRAFT" are adressed and in in line #3 it says the action is "mandatory" but in section "A" it says "Aircraft affected by this inspection are ... ALL BAe 146 100, 200, 300 Series". I might be not the sharpest knife in the drawer but it's still not clear for me.