If you're asking about coming back to a location after you have
finished to do additional shots you will simply continue with the
letters. For example; you finished scene 28 on Friday - 16 set
ups - and you return to the set on Monday to pick up a few shots
you missed. You will start with 28R and then 28S and so on.
If you are asking about starting (say) shot 28C which is eight
lines on line 5 to 8 then you will use 28C-PU and mark the script
accordingly.