Yarn AB? Help!

Hi all,

I'm starting on Ratty by Claire Garland - a little out of my comfort zone but he looked cute. The pattern is telling me to cast on yarn AB but hasn't mentioned what that means anywhere and doesn't have any instructions on what to do with these two (???) colours. Does it mean I cast on with both yarn A and B and knit the whole thing holding them together?

Very grateful for this sub and everyone's help, finished my cardigan thanks to you!