Q:

How to build with different setting for a test category on TFS 2017

I have a situation where my project needs to be tested in x64, but now I'm adding Coded UI tests which need to run in 32 bits. 
I have previously used the multi-configuration option to spread work across multiple agents

The problem is that adding another variable for the test settings file will cause my build to run all categories in both x64 and x86 test settings and I want to be able to choose which categories use which settings, how can I do that? 

A:

No, you can't.  Seems you want to achieve only build specifically configuration and platform such as Debug|x86 , Release|x64.  
You cannot specify unique combinations of variables.  For now, vNext Build will always build all permutations of the variables specified. 
You could add a uservoice here, TFS admin and PM will kindly review your suggestion.