If you have money on your card, you can use it for several people simultaneously. I wonder how many people actually use this feature. (Probably should ask HKL/YTV.)
This is what makes correcting mistakes so difficult. The first time some "helpful" person took some money needlessly off my card, I went to a HKL office and demanded it back, and they told me that they cannot return it, even though they can see that transaction, because a card can be used by several people at a time and they have no way of making sure there was only one of me. They returned the money anyway after I looked at them. Must've been one hell of a look.