Four directions, one idea
For an internal feature at MMC the tolerance is subtracted from the MMC size, giving the smallest inner boundary the hole can present. For an external feature at MMC it is added, giving the largest outer boundary the pin can occupy. At LMC both directions reverse.
The idea underneath is the same each time: take the material condition the frame names, and move the boundary in whichever direction makes the feature worse. Memorising four formulas is fine; understanding the one rule is better, because it survives the case the formulas do not cover.
Why there is none under RFS
A virtual condition is constant only because the geometric tolerance is tied to a fixed material condition. When the tolerance applies regardless of feature size, the stated allowance applies at every produced size and no boundary stays put.
Software that prints a virtual condition in every mode teaches a boundary the drawing never created. The honest behaviour is to show nothing and say why, which is what DatumLab does.
Fixed fastener checking in one line
For a pin entering a hole, the pin outer boundary must not exceed the hole inner boundary. Compute the virtual condition of each from its own frame and compare. If the pin comes out at or below the hole, the worst case pair assembles and every better pair has clearance.
This is why both frames in a mating pair normally carry the MMC modifier. It is not a habit; it is the condition that makes the comparison valid.
When the resultant condition is the one you want
The resultant condition is the boundary at the opposite extreme: the far size limit plus the tolerance plus the full bonus available there. For a hole toleranced at MMC that is the largest space the hole can consume.
Use it when something has to sit beside the feature rather than inside it: a wall, a rib, a neighbouring hole. Choosing the wrong boundary here is a design error rather than an inspection one, and it does not show up until parts exist.
A 12.00 mm hole at MMC with a 0.30 mm position tolerance
The hole is toleranced diameter 12.00 to 12.30 mm with position diameter 0.30 mm at MMC.
- Virtual condition, the inner boundary
- 11.700 mm
- Resultant condition, the outer boundary
- 12.900 mm
- Largest pin guaranteed to assemble
- 11.700 mm
- Virtual condition of a mating 8.00 mm pin with 0.12 mm at MMC
- 8.120 mm
The gauge pin for this hole is made at the virtual condition and held at true position. A hole that receives it has satisfied size and position together, which is the whole appeal of the MMC modifier.