Understanding Overriding ionic variables

hi , perhaps this is a silly question but I am a bit confused.

what the difference between $tabs-background and $tabs-md-background?
does that mean if I use the second one or maybe the first one , it will apply for both Android and IOS ?

and also some of them come with md and ios… for example, $tabs-ios-tab-icon-color-active
but there is no $tabs-tab-icon-color-active .

I guess what I am trying to understand, which one applies for both of the platforms… Do I have to use both md and ios in order to get it working for both platforms ?