For (linear) posit, what the "last place" is varies. Versus a fixed-size significand, there is no 0.5 ulp guarantee. If you are in the regime of full precision, then there is a 0.5 ulp guarantee. The rounding also becomes logarithmic rather than linear in some domains (towards 0 and +/- inf), in which case it is 0.5 ulp log scale rather than linear, when the exponent scale is not 0.

For my log numbers under ELMA (with or without posit-ing), the sum of 2 numbers alone cannot be analyzed in a simple ulp framework I think, given the hybrid log/linear nature. Two numbers summed are both approximated in the linear domain (to 0.5 ulp linear domain, assuming alpha >= frac + 1), then summed exactly, but conversion back to the log domain when done is approximate, to 0.5 ulp in the log domain. But the result is of course not necessarily 0.5 ulp in the log domain. Multiplication, division and square root are always the exact answer however (no rounding). The sum of two log numbers could of course also be done via traditional LNS summation, in which case there is <0.5 ulp log domain error.

Kulisch accumulation throws another wrench in the issue. Summation of many log domain numbers via ELMA will usually be way more accurate than 0.5 (log domain) ulp rounding via LNS traditional summation techniques, because the compounding of error is minimized, especially when you are summing numbers of different (or slightly different) magnitudes. Kulisch accumulation for linear numbers is of course exact, so the sum of any set of numbers rounded back to traditional floating point is accurate to 0.5 ulp.

The rounding errors for linear posit are exact numbers (excepting division), assuming higher precision. The rounding errors for LNS add/sub are not exact numbers in the representation in the general case. 2 and sqrt(2) are represented exactly, but (2 + sqrt(2)) is not.