Model-Based Definition > Geometric Dimensioning and Tolerancing Advisor > GD&T Advisor > Establishing DRF (ISO) > Datum Feature Material Requirements (ISO)
  
Datum Feature Material Requirements (ISO)
For a datum reference frame in which a referenced datum feature is a feature of size, you must specify a material requirement (MMR, LMR, or RFS) for that datum feature. Keep in mind that MMR and LMR introduce datum feature displacement to the datum reference frame.
The use of a datum feature letter in a tolerance frame, along with the material requirement of the size datum feature, provides a clear understanding of how the datum feature is to be simulated.
MMR, Ⓜ, is placed after the datum letter in the tolerance frame to establish a fixed datum size simulation at the MMVC of the datum feature.
LMR, Ⓛ, is placed after the datum letter in the tolerance frame to establish a fixed datum size simulation at the LMVC of the datum feature.
RFS is used to establish a variable datum simulation from the largest inscribed or the smallest circumscribed size of the datum feature.
Note that you may 'Accept' the DRF definition without specifying a value for a datum feature material requirement (i.e., leave the value set to '?') if you want to defer that decision until later. If you do so, there will be a 'DRF specification is incomplete' advisor message associated with the DRF until that value has been specified.