Hello,
I have also been working on this topic for some time. When training outside, the training control via CTL works very well and gives me a good feedback of my load and recovery.
When training indoors in winter on the smarttrainer, it looks completely different. Because the training stimuli from shorter and harder units via CTL are only very slightly represented here, a different metric was necessary. I was able to display the desired representation via CIL. Here the IF is used instead of the TSS as indicator.
Especially with short and hard VO2max units the CTL does not show the performed load. The result is that the fitness stagnates or even develops negatively and you will be tempted to increase the volume and the load further, which inevitably leads to overloading. I had to make this experience myself last year.
With the additional observation of the CIL I can recognize the actual load during winter training indoors much better and can plan and control the loads more precisely.
Sorry for the strange English but this explanation was not possible without a translator. 
