There is only limited knowledge about the main engine, which basically serves as client and, if necessary (e.g. in single player) as server, or, alternatively, as dedicated server.
It is believed that the main engine, if run as client only, and the refresh DLL's do not require the Game DLL at runtime. There is a certain amount of overlap between the refresh libraries and the Game DLL, which indicates either copies of sources, or compile time access to the GameDLL. See the documentation of the Rendering/Refresh Libraries for a list of shared functions.
The following description is based on strings as found in the main executable, and guesses.
According to John Carmack, the Zone Memory allocation
as used in DOOM was the only module that had a chance
of being used for Quake. See the DOOM source,
for details of the previous implementation.
m_cursor m_main_game m_main_multiplayer m_main_options m_main_video m_main_quit m_main_plaque m_main_logo m_banner_multiplayer m_banner_game m_pitch m_yaw m_forward m_side
menu_main menu_game menu_loadgame menu_savegame menu_joinserver menu_addressbook menu_startserver menu_dmoptions menu_playerconfig menu_credits menu_multiplayer menu_video menu_options menu_keys menu_quit
svc_frame svc_deltapacketentities svc_packetentities svc_playerinfo svc_download svc_centerprint svc_spawnbaseline svc_configstring svc_serverdata svc_stufftext svc_print svc_sound svc_reconnect svc_disconnect svc_nop svc_inventory svc_layout svc_temp_entity svc_muzzlflash2 svc_muzzleflash svc_bad svc_lightstyle
cl_testblend cl_testparticles cl_testentities cl_stats cl_stereo_separation cl_stereo cl_blend cl_lights cl_particles cl_entities cl_gun cl_footsteps cl_noskins cl_autoskins cl_predict cl_maxfps cl_upspeed cl_forwardspeed cl_sidespeed cl_yawspeed cl_pitchspeed cl_anglespeedkey cl_run cl_shownet cl_showmiss cl_timeout cl_testlights
SV_Multicast SV_StartSound SV_StartSound SV_StartSound SV_SendClientMessages SV_Configstrings_f SV_Baselines_f SV_Begin_f SV_ReadClientMessage SV_AreaEdicts SV_FatPVS SV_ReadPackets