Here’s a technical explanation of resultant condition. Note that this explanation is in the context of ASME GD&T, but also applies to ISO GD&T (in GPS), with some caveats. Believe it or not, this detailed explanation is simplified! Understanding GD&T requires knowing and understanding layers of terms, definitions, and concepts, as well as the context for a specification.
Simply put, the resultant condition is the state of a toleranced feature at the other boundary, the boundary the design engineer believes they care less about. Obviously, the design engineer must be cognizant of both the MMB and LMB for every feature, but in many cases, there is one boundary that is more functionally significant.
ASME Y14.5-2009 and prior versions defined the virtual condition of a feature as the collective effect of the nominal feature definition (specified size and shape) plus or minus the allowable form, size, orientation, and location variation as applicable. Whether the allowable variation is added or subtracted is a function of whether the toleranced feature is an external or internal feature. According to ASME Y14.5-2009, a virtual condition only exists if a feature is controlled by a geometric tolerance applied with an MMC or LMC modifier. This is an unfortunate technicality, as the concept could be and should be expanded to other cases. But, the definition is what it is.
The idea is that whichever material condition modifier is specified for the geometric tolerance, MMC or LMC, there is a virtual condition in the associated boundary, calculable by the appropriate addition or subtraction.
The resultant condition is the other boundary. E.g., if a hole is defined by a directly-toleranced dimension and controlled by a positional tolerance with an MMC modifier, perhaps one of the most common use cases, there is a resultant condition that corresponds with the LMC for the hole.
The envelope principle in ASME obfuscates some of the issues at play, namely that the allowable form error specified for the feature is also part of the calculation. Since, in ASME the envelope principle is the default, we sometimes forget the form error and how its limits affect the calculation.
For a part dimensioned and toleranced in accordance with ASME Y14.5, the resultant condition is either
- the sum or difference of the size, size tolerance, the geometric tolerance at the specified material condition, and the additional (bonus) tolerance for the opposite material condition, or
- the sum or difference of the size, size tolerance, the geometric tolerance at the specified material condition, and the allowable form error at the other material condition, or
- some combination of these.
To reinforce the last paragraph, the full additional (bonus) tolerance is only allowed (and in spec) if the toleranced feature has perfect form at the other size (the non-specified size). E.g., if a hole has a positional tolerance applied with an MMC modifier, the only way that the full additional (bonus) tolerance is allowed and in-spec is if the hole has perfect form at LMC, which is not required. If the hole has any form error, the amount of additional (bonus) tolerance is decreased by that form error. The resultant condition worst-case boundary remains the same (it has the same value), but the variation that leads to that boundary differs.
These concepts are clarified in section 5.1 Boundaries (pages 52-62) in my book “The GD&T Update Guide: ASME Y14.5-2009” which includes many detailed examples of maximum material boundary (MMB) and least material boundary (LMB). [Figure 5-5 from "The GD&T Update Guide" is shown above.]