TPL121800 or TPL121600?

The case is the same, so the decision is purely about reserve and cost. The 121800 gives 175 Ah at the 8 hour rate against 159, and 535 watts per cell at 15 minutes against 445, for 0.66 lbs more. Where the rack position is already committed and the reserve target has crept up, the larger block is the straightforward answer. Where the site comfortably meets its target today, the 121600 is the right specification and we will quote it that way.

What does the 1800 in the part number mean?

It is a nominal amp hour designation applied at the catalog level. CSB publishes two measured ratings, 192 Ah at the 20 hour rate and 175 Ah at the 8 hour rate, and 180 sits between them. Which side of it you land on depends entirely on how hard you draw the block. Size from the column that matches your discharge duration and treat the badge as a catalog label.

Will it fit a 19 inch rack?

No. The block is 21.54 inches long and CSB classes it as a 23 inch front terminal part, as it does the TPL121600 and the TPL122000. For a 19 inch relay rack the TPL121000 is the corresponding part at 20.16 inches long and 100 Ah on the 8 hour rate. Confirm the rack width before ordering, because the two families do not interchange.

Why is internal resistance higher than on the smaller TPL121600?

That is what CSB publishes: 3.90 milliohms here against 3.70 for the 121600, both quoted as approximate, and short circuit current of 2,575 amps against 2,593. Every discharge column runs the other way, with this block ahead at every duration from 15 minutes to 20 hours. We report both figures as printed rather than dropping the two that read less well. If your design turns on the fault current figure, ask a specialist to confirm it with CSB in writing.