Below are definitions of some of the notable data structures found in Mission Impossible (N64).

Burp headers describe a series of compressed and/or uncompressed data blocks in ROM. Compressed data blocks are encoded using the DEFLATE algorithm.

struct BurpHeader
{
    char   signature[4];  // "Burp"
    u32    numBlocks;
    struct BurpBlock blocks[];
};

struct BurpBlock
{
    u32 dstSize;
    u32 srcSize;
    u32 unk08;
    u32 offset;  // relative to start of BurpHeader
    u8  type;    // 0x00 if uncompressed, 0x04 if compressed using headerless DEFLATE
    u8  _pad[3]; // structure padding
};

struct Object {
    /*0x00*/ void *unk00;
    /*0x04*/ void *unk04;
    /*0x08*/ u32   unk08;
    /*0x0C*/ u32   unk0C;
    /*0x10*/ u8    unk10[2];
    /*0x12*/ u16   unk12;
    /*0x14*/ u16   unk14;
    /*0x16*/ u16   unk16;
    /*0x18*/ vec3f position;
    /*0x24*/ u8    unk24[0x0E];
    /*0x32*/ u16   unk16;
    /*0x34*/ u8    unk34[0x0C];
    /*0x40*/ vec3f rotation;
    /*0x4C*/ s32   timer;    // copied from the VI counter
    /*0x50*/ u8    unk50[2];
    /*0x52*/ u16   unk52;    // some flags? unsetting bit 5 makes object invisible
    /*0x54*/ s32   unk54;    // maybe an animation counter, but locking it seems to have no effect
    /*0x58*/ u8    unk58[4];
    /*0x5C*/ s32   unk5C;
    /*0x60*/ struct Object *unk60;
    /*0x64*/ u8    unk64[4];
    /*0x68*/ s16   unk68;
    /*0x68*/ s16   unk6A;
    /*0x6C*/ s32   unk6C;
    /*0x70*/ u8    unk70[8];
    /*0x78*/ u16   unk78;
    /*0x7A*/ u16   unk7A;
    /*0x7C*/ u8    unk7C[4];
};

/*80086190*/ struct Object *gLevelObjects;
/*80093840*/ s16 gNumLevelObjects;

Below is the header struct for level object placement data. When a level is loaded, one of these structs is decompressed from ROM along with ObjectPosRot and ObjectPosArray structs that follow. All pointer members of this struct are initially offsets relative to the beginning of the struct, but the game converts them to virtual addresses in-place.

struct ObjectPlacements
{
    /*0x00*/ s32    numPlacements0;
    /*0x04*/ struct ObjectPosRot *placements0;
    /*0x08*/ s32    numPlacements1;
    /*0x0C*/ struct ObjectPosRot *placements1;
    /*0x10*/ s32    numPlacements2;
    /*0x14*/ struct ObjectPosRot *placements2; // unconfirmed type
    /*0x18*/ s32    numPlacements3;
    /*0x1C*/ struct ObjectPosArray *placements3[];
};

struct ObjectPosRot
{
    /*0x00*/ u8    unk00[2];
    /*0x02*/ vec3s position;
    /*0x08*/ vec3s rotation;
};

struct ObjectPosArray
{
    /*0x00*/ s32   numPositions;
    /*0x04*/ vec3s positions[];
};

/*0x800A98CC*/ struct ObjectPlacements *gObjectPlacements;

Breakdown of object placement data for the first mission: https://pastebin.com/raw/657rL7jD