R

rad_to_deg
radial
radioButton
radioButtonGrp
radioCollection
radioMenuItemCollection
rampColorPort
rand
randomizeFollicles 
randstate
rangeControl
readTake
rebuildCurve
rebuildSurface
recordAttr
recordDevice
redo
reference
referenceEdit
referenceQuery
refineSubdivSelectionList
refresh
refreshAE 
refreshEditorTemplates
regionSelectKeyCtx
registerPluginResource 
rehash
relationship
reloadImage
removeJoint
removeMultiInstance
removePanelCategory 
rename
renameAttr
renameSelectionList 
renameUI
render
renderer
renderGlobalsNode
renderInfo
renderLayerParent 
renderLayerPostProcess
renderLayerUnparent 
renderManip
renderPartition
renderPassRegistry
renderQualityNode
renderSettings
renderThumbnailUpdate
renderWindowEditor
renderWindowSelectContext
reorder
reorderContainer
reorderDeformers
reparentStrokes 
requires
reroot
resampleFluid
resetAE 
resetPfxToPolyCamera 
resetTool
resolutionNode
resourceManager
retarget
retimeKeyCtx
reverseCurve
reverseSurface
revolve
rgb_to_hsv
rigidBody
rigidSolver
roll
rollCtx
rootOf 
rot
rotate
rotationInterpolation
roundConstantRadius
rowColumnLayout
rowLayout
runTimeCommand
runup

indicates a MEL script