The Att in the Clients (Att*) stands for Attendance. In the Operator Report where you see something like Clients (Att*) being 3(7), it means that there were 3 unique clients, but some of them came in multiple times, so the attendance is 7.
The Att in Avg $/Client (Att*) is referring to the average client spend based on the number of times the clients came in (their attendance), for the date range you have selected.
For example, if Clients (Att*) is 1(2) and Avg $/Client (Att*) is $300 ($150), what this is saying is that the client came in twice during that date range and spent a total of $300 during both visits. $300 divided by 2 visits is $150 — so it is saying that on average the clients spent $150 per visit.