The MD3 Export Options window is displayed when you save a model in Quake MD3 format. See Quake MD3 for more details about player models and animation.cfg.
The Save as a Player Model checkbox allows you indicate if you want to export as a Quake 3 composite player model (head.md3, upper.md3, and lower.md3 models). If it is grayed out, see the list of requirements for export on the MD3 page.
The Save animation.cfg checkbox allows you to indicate if you want to write "animation.cfg" file along side the model. This file contains a list of animations with looping and frames per-second information and can contain other arbitry data from MD3_CFG_* meta data.
Click Ok to save the model with the specified options or click Cancel to ignore any changes.