RB in National Index of Parish registers: Suffolk?

[Birtle]

I'm just delving into Anthony Wilcox's Suffolk volume in the National Index of Parish Registers and am utterly flummoxed. What does RB stand for? It's not in the lists of abbreviations, and my imagination is just not up to the challenge (she said sheepishly)! It appears consistently between OR (Original parish registers) and BT (Bishops Transcripts). Can anyone enlighten me please?

[Birtle]

Ah ha! Register Bills, I reckon. https://www.suffolkarchives.co.uk/wp-content/uploads/2023/06/Ancestry-FAQ-Final.pdf - which is worth a read in itself