It has been previously shown that any measurement system specific relationship (SSR)/ mathematical-model Y_d = f_d ({X_m}) or so is bracketed with certain parameters which should prefix the achievable-accuracy/ uncertainty (e_d^Y) of a desired result y_d. Here we clarify how the element-specific-expressions of isotopic abundances and/ or atomic weight could be parametrically distinguished from one another, and the achievable accuracy be even a priori predicted. It is thus signified that, irrespective of whether the measurement-uncertainty (u_m) could be purely random by origin or not, e_d^Y should be a systematic parameter. Further, by property-governing-factors, any SSR should belong to either variable-independent (F.1) or -dependent (F.2) family of SSRs/ models. The SSRs here are shown to be the members of the F.2 family. That is, it is pointed out that, and explained why, the uncertainty (e) of determining an either isotopic abundance or atomic weight should vary, even for any given measurement-accuracy(s) u_m(s), as a function of the measurable-variable(s) X_m(s). However, the required computational-step has been shown to behave as an error-sink in the overall process of indirect measurement in question.
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