This should be styled roughly as follows:

t = { a <tab> for ..., \n b <tab> for ..., and \n c <tab> for ... . }

(if you see what I mean)

You mean for all such definitions by cases in the document (like the other one in the screenshot)?

After all, I wouldn't want to introduce inconsistency with the definition by cases in the very next item.

No, just in this case. I don't want the second column to start with a dangling comma.

In pseudo-latex:

t = \begin{cases} 1 & \text{for $a = 1$,}\\ 2 & \text{for $a > 3$, and}\\ 3 & \text{otherwise.} \end{cases}

just to demonstrate how to integrate sentence punctuation into cases.