Does the MCS reading on Wireless Wires reflect TX or RX modulation?

It should as it is standardized. Check this table.

It should as it is standardized. Check this table.

But which direction? Transmit or receive?

It should as it is standardized. Check this table.

But which direction? Transmit or receive?

Both. If parties don't agree about MCS, connection fails. So RX side can report correct MCS as well.

Even though it's TDD, MCS can be different for TX and RX. One example why this can happen is interference source close to one party.

It should as it is standardized. Check this table.

But which direction? Transmit or receive?

Both. If parties don't agree about MCS, connection fails. So RX side can report correct MCS as well.

Even though it's TDD, MCS can be different for TX and RX. One example why this can happen is interference source close to one party.

What if the AP reports MCS 8 while the station reports MCS 6? They don't agree, but the connection has not "failed". Which side has the interference (assuming that's the problem)?

Most of us, forum members, don't have wireless wire devices at hand. If you post some screen shots with said readings, we can speculate further.

My point is: if one device only reports one MCS, then it might be only one direction ... and slightly more meaningful (to me at least) would be to report MCS for TX. As link state varies over time (even quite rapidly), that value might be an average (or median or any other statistical value) over period of time. Even if both devices would report MCS for both directions, they might report different values from time to time due to different reporting time interval start times.

On the other hand, if the example in manual is correct, then MCS is displayed among other RX-related stuff, TX-related stuff is groupped at the end of printout.