A system logs three events in a specific order: first 'login', then 'query', and finally 'logout'. Which of the following correctly represents this finite, ordered series of events using the notation for a sequence of elements?

Analyzing Sequence Notation

A data scientist is recording the unique user IDs of everyone who visited a website on a particular day. The order in which the users visited is not important; the goal is only to have a final collection of unique visitors. The data scientist represents this collection as $\langle \text{ID}_1 \rangle, \langle \text{ID}_2 \rangle, \dots, \langle \text{ID}_\kappa \rangle$. Why is this an inappropriate use of this notation for this specific task?