The other day I was coming back from ZUR (in Y) and, unusually for my SH flights, I was checking in hold baggage. I also had my usual hand baggage with me, namely one medium/large rucksack type bag. It usually fits under the seat in front, although when fully bloated this can be a struggle. I am also quite happy to slot it under the seat (which also means that I tend to be quite relaxed about getting on board early, as I'm not too worried about the potential lack of overhead storage space), but it it's not a small bag, and does therefore take up all of the space under the seat in front

At the check in desk, having deposited my suitcase, the assistant then tagged said hand luggage with the yellow tag, which states that the bag must be placed under the seat in front. I always thought that you were allowed one cabin bag and one personal item, and that it is the latter that gets the yellow tag. However I only had the one piece of hand luggage. This got me wondering, what are the rules? Is it entirely at the discretion of the check in/gate agents? Or is the rule as I had always understood it, as set out above?