t required argument is a number that identifies the tab
(say, "3").  The second required argument is an indent from the
left margin (say, 6 picas).  The third required argument is the
length of the tab (say, 3 picas).  Therefore, at a minimum, when
using this macro, you would enter:
<br/>
<span class="pre-in-pp">
  .TAB_SET 3 6P 3P
</span>
The remaining two arguments are optional.  The first is a
single letter, either <kbd>L, R, C</kbd> or
<kbd>J</kbd>.  The second, which is itself
optional after <kbd>L, R,