T = Taxpayer

S = Spouse

And Taxpayer is always the person listed first on the tax return and the spouse is the person listed second.  

On the 1040 form it shows "Your SSN" and "Spouses SSN"

Form 8889 is for reporting transactions on HSA accounts.  Note that HSA accounts are individual accounts, so TurboTax provides separate forms for taxpayer and spouse so that each one's HSA activity is reported individually when a married couple is filing jointly.