I have been a Xert subscriber for about 4 years and in all that time my TL has always been about 10 more than CTL (as measured in WKO5).

No problem with that. The XSS of a ride is generally a few more than the TSS of the same ride. Note that normally my FTP and TP are very similar.

The oddity is that in the last 2 months TL=CTL.

I think I know the answer as my FTP has reduced but not my TP. My last BT was 2 months ago. I am putting hard efforts in, yesterday I did 30 minutes at 105% FTP which then recommended an increase by 2-3 watts but would still have been 10w below current TP

As I am writing this I think I have found the answer. I had set my decay rate to slow. So TP had stayed artificially high. I have just amended to optimal decay and recalculated and TP has dropped by 4w

I therefore expect in the next couple of weeks that the gap between CTL and TL will increase up to around the 10 mark again.

Leaving this in as it might help someone else. Also I have typed it all in!!!.

Great question. Both are analogous in terms of what they’re trying to accomplish - that is, create a numerical value that represents how much training you have historically done.
The basics:
CTL is 42 EWMA of TSS
TL is 60 day EWMA of Low XSS + 22 day EWMA of High & Peak XSS

TSS is calculated on time & some weighted average power during activities. It can only be accrued at 100/hr, assuming a full hour at FTP.
XSS is based on work performed, relative to MPA. In theory, it doesn’t have a limit in terms of XSS/hr. Practically, I’d say around ~200 XSS/hr is the highest achievable value.

For most sub-(F)TP efforts, there’s likely a very linear relationship between XSS & TSS, but this changes as efforts above (F)TP are added in. This is because Xert awards more XSS when work is performed under fatigue, unlike TSS. I often explain it like this… a 20 min FTP test with other systems awards the same TSS for the first minute of the 20 min test as the last. If you’ve ever done an FTP test, you know the last minute of that test is much harder than the first - and this is because you’re becoming limited by your MPA. Xert knows & understands this, so you’ll receive far more XSS in the last min of that 20 min test than you do for the first minute of the test.

TLDR: they’re both numbers that represent how much training you’re doing, but it’s sorta a little bit of apples to oranges, since they’re both calculated differently.