What is * * * in zcml used for

In the docs I noticed that ZCML registration uses

for="* * *" 

What is the use of that (is it the same as "*" ?


No, this is not the same.
3 * means a multiadapter adapting 3 components context, request, field.
one * means zope.interface.Interface


Thanks for you answer.

(Sorry for my ignorance): In this (javascript) example: What would be the use case for * * * ( would it not be better to register it for an-addons browser layer ?)

Yes indeed, you can use for="* yourpackage.interfaces.ILayer *" to register it for your addon brower layer. It should work.