Splitlevel in TTree Branch for class with custom streamer

I noticed (some time ago) in the Reference Guide, for the method TTree::Branch for a class, the following comment:
"Use splitlevel < 0 instead of splitlevel=0 when the class
has a custom Streamer"
Can somebody tell me why ? What happens if you leave splitlevel=0? And can you use any value of splitlevel<0? Does it split the branch the same as splitlevel>0 for “normal” objects ?
Thanks a lot

Hi John,

In principle splitlevel <0 is redundant with the new versions.
The negative case was used when ROOT was not able to detect
automatically classes with a custom Streamer.
With the recent versions, specifying splitlevel=0 is sufficient
to force a branch to not be split.