Where should I enter custom field values to be updated: in All Language Pairs, or under the specific language pair?

Hello,

I've been using Studio for a couple of years (currently version 2017) and am fairly familiar with its settings, but one thing that is still unclear to me is the difference between the settings under 'All Language Pairs' and those under the specific language pair (in Project Settings, not Options). More specifically, this post is about where I should enter the values I want to assign to my custom fields: in All Language Pairs, or in the specific language pair?

This post by  (which contains lots of helpful tips!) suggests checking 'Update' under 'All Language Pairs', but entering the actual field values under the specific language pair - can anyone explain why it is necessary (or recommended?) to do this, instead of entering the field values under 'All Language Pairs'?

I am wondering whether for projects containing a single language pair (= all of my work), this actually makes no difference, but my understanding of these settings is too hazy to be sure, so if any more experienced Studio users can shed any light on this, I'd be very grateful!

Thanks,

Hayley