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Often two parallel holes are used as datum features. The first figure shows the desired datum planes. There are a couple of ways to accomplish this. The default for datum feature simulators is that they are located at the basic location indicated on the drawing regardless of whether they are applied at RMB, MMB or LMB.
Here datum features A and B apply regardless of material boundary because there is no modifier following the datum feature reference in the feature control frame. The simulator for datum feature B is fixed at the 100 basic location, parallel to the axis of A and should expand (progress from MMB toward LMB) to make maximum possible contact with datum feature B. It is likely, that in this case, the simulator will only contact datum feature B at one point and may not do a good job of orienting the datum planes. Note: AME in these figures means actual mating envelope.
Adding the translation modifier to the datum feature B reference means that the datum feature simulator may float in location relative to the datum axis established by datum feature A. This will usually establish a more stable datum reference frame.