Can someone help me understand why there are multiple signals listed for registered wireless clients? See attached image.

Chains 0, 1, and 2 (Ch0, Ch1 & Ch2) are antennae on a single radio. If your radio is 2x2 MiMo then you have two antennae (Ch0 and Ch1).

TX/RX Signal Strength = Ch0+Ch1+Ch2
TX/RX Signal Strength Ch0 = Signal from antenna connected to chain 0
TX/RX Signal Strength Ch1 = Signal from antenna connected to chain 1
TX/RX Signal Strength Ch2 = Signal from antenna connected to chain 2

In the world of decibel, -20dBm + (-20dBm) = -17dBm

The values displayed in the picture are TX values only. This is the situation when you are connecting to non ROS system, the router can only display the TX signal strength from you radio.

The Ch2 field is empty because your radio is 2x2 MiMo with only two chains.

Ok so now I'm a bit more confused.

My image shows a wireless router that has five (5) readings to a single wireless client.

Why would I have five readings for a router that has two chains...2x2?

This is signal strength level at different rates together with time how long were these rates used. Nothing to do with MIMO.

Does the usage time overlap for these connections?

In other words, does the AP have multiple simultaneous connections to a client to allow for more bandwidth?

Read post number 2.