What does the lowest internal resistance in the range buy me?

Two things. Under load it means less voltage sag for a given current, so the string holds its terminal voltage longer into a discharge, which is where the higher 15 minute power figure comes from. For maintenance it sets the baseline that a conductance or impedance program watches over the years; a block that starts at 3.60 milliohms has further to drift before it crosses the threshold your program treats as end of life. It does not change the discharge ceiling, which stays at 800 amps with the other I2 blocks.

Why is the N·m torque tolerance different on this datasheet?

CSB's 2026 sheet for this part prints 5.1 ± 1.0 N·m where the 12550, 12800 and 121000 sheets print 5.1 ± 0.6. All four give the same 44.9 ± 9.0 lbf-in, and 9.0 lbf-in converts to about 1.0 N·m, so this sheet's metric tolerance is the one that agrees with the imperial figure. Work in lbf-in and the discrepancy disappears. Either way the target is the same and the window is wide enough that a small torque driver set to 45 lbf-in is inside it.

Should the protection study use 2,648 amps?

Yes, and for a series string it is the per-block figure that drives the fault current, because the blocks are in series and share the same current path. A DC fuse or breaker upstream of a 48 volt string of four must be rated to interrupt that current at the string voltage. It is the highest short circuit figure in the XTV range, which is why this block is the one to name explicitly when the study is being written, even if the final selection lands on a smaller size.

What does the XTV range not do?

It is not built for deep cycling and it is not a high rate UPS block. CSB rates it for float service outdoors: sit at 13.5 to 13.8 volts, deliver when the utility drops, recharge, repeat. The discharge tables run out to 20 hours because that is the duty. A load that cycles the string daily belongs on a battery rated in cycles, and a 15 minute UPS transfer belongs on the HR or HRL ranges rated in watts per cell. This block is for the cabinet that has to make it to morning in January, and that is a different job from either.