I have been at two exams recently with different invigilators and
no problems with highlighting (as per very first sentence in Stuart's post)
But no writing, notes, references (e.g. arrows), nothing other that highlighting is allowed. Number of tags was counted and every tag must have a word from the topic name on the page, or page number e.g. 1.5 (as in ENR 1.5).
My only "sin" was tag named "OCTAS" describing clouds, because that wasn't the topic name. I happily removed that tag as I already memorised the section number.
I must admit I have never seen so passionate invigilator checking thoroughly
almost every page from AIP, ERSA, DAPs, CAR/CASR extract, all charts including PCA, even e6b was turned around
She did that with
every candidate and even made a couple of phone calls to someone seeking for clarifications.
Exam started ~40 mins later than scheduled.
Relax, nothing wrong with highlighting.