Which build of VP15 do you use?

The latest one, built 261

I never have been able to customize the constant bitrate to 6000 kbps, also not in Vpro 14 with f.i. the Main Concept AVC codec during the Sony era.

Always could and can in VPro15 set the variable bitrate with max and average both to 6000 kbps with the same result.

Aha, I never thought that setting the variable bit rate with max and average to the same number would be similar to constant bit rate!

Try it, maybe it is not precise constant 6000 but the avarage will be 6000.
You only can have a fixed number if you choose for Main Concept and 2 pass rendering.

I just tried it, and MediaInfo still report it as variable .. so dose this have any impact when playing the file on low end smartphones, when compared to a real constant file?

I might be mistaken, but I always had the impression that variable bit rate files require more processing power to play than constant bit rate files!

Actually for codecs like H.264, H.265, VP9 and some others a constant bitrate does not make too much sense. You'd trash nearly all of its effency with using a constant bitrate.

That said, I was mistakenly thinking using custom bitrates would work for Magix AVC and constant bitrate but this isn't true like Cornico already mentioned. You'd better use a variable bitrate and different values for average and peaks else limiting such a codec so much.

OK, in this case I will just use variable bit rate.

But I am wondering, if variable bit rate is better, then what is the point and advantage of constant bit rate, if any?

Then why that option is still available if there is no benefit for it? It just dose not make any sense! MAGIX should remove it!

But Mainconcept AVC is enabled by default!

I think Cornico means the separate MainConcept AVC render format which is hidden in VP15 by default, while you probably mean that MainConcept selection within the Magix AVC render window. These are different things/places.