MDL
MDL is the extension for Source's proprietary model format. It defines the structure of the model along with animation, bounding box, hit box, material, mesh and LOD information. It does not, however, contain all the information needed for the model. Additional data is stored in PHY, ANI, VTX and VVD files, and sometimes, usually for shared animations, other .mdl files.
File format
Some details of the file format may be gleaned from the source code in Valve's studio.h, specifically the struct studiohdr_t. The early header defines a series of offsets and lengths for various sub-sections within the file, along with some key scalar information. The MDL also contains the names of materials (VMT) which are used as the numerically-defined swappable skins.
Main header
struct studiohdr_t
{
int id; // Model format ID, such as "IDST" (0x49 0x44 0x53 0x54)
int version; // Format version number, such as 48 (0x30,0x00,0x00,0x00)
char[64] name; // The internal name of the model, padding with null bytes.
// Typically "my_model.mdl" will have an internal name of "my_model"
int dataLength; // ??
// A vector is 12 bytes, three 4-byte float-values in a row.
Vector eyeposition; // Position of player viewpoint relative to model origin
Vector illumposition; // ?? Presumably the point used for lighting when per-vertex lighting is not enabled.
Vector hull_min; // Corner of model hull box with the least X/Y/Z values
Vector hull_max; // Opposite corner of model hull box
Vector view_bbmin;
Vector view_bbmax;
int flags; // ??
// After this point, the header contains many references to offsets within the MDL file and the number of items at those offsets.
int bone_count // Number of mstudiobone_t structs
int bone_offset // Offset of first mstudiobone_t struct
int bonecontroller_count // Number of mstudiobonecontroller_t structs
int bonecontroller_offset // Offset of first mstudiobonecontroller_t struct
}