In addition to the generic FME feature attributes that FME Workbench adds to all features (see About Feature Type Attributes), this format also adds format-specific attributes (Format Attributes).
TetGen features consist of geometry and attributes. The attribute names are defined in the DEF line and there is a value for each attribute in each TetGen feature.
Each TetGen feature contains a special attribute to hold the type of the geometric entity. At present, only solids are supported. These are identified by the format attribute tetgen_type with a value of tetgen_solid.
Features can have one or both of the format-specific attributes tetgen_region_attribute and tetgen_volume_constraint. Both of these attributes store float values, and both are optional.
See the TetGen documentation for more information on region attributes and volume constraints.