FTE's delivery of the entities a program sends itself (SendEntity) to the client programs of the players watching (sv_ents.c).

server = QuakeServer.Network.watch(server, 1)
{%{1 => updates}, server} = QuakeServer.Network.frame(server, [1])
updates #=> [{:update, 37, <<...>>}, {:remove, 40}]

frame/2 runs once a server frame (SV_SendClientMessages). First every watching player keeps the bits of each live entity's SendFlags (SV_ProcessSendFlags). Then each player being sent a packet is first given the sounds and particle effects it lacks (QuakeServer.Precaches), then its updates (SV_EmitCSQCUpdate): for each entity with a SendEntity the player may see, in order, that function runs with self the entity and as parameters the player and the kept bits in three 24-bit pieces (an entity the player does not have gets every bit), and what it writes to MSG_ENTITY is the update when it returns true. An entity the player has is removed when its function returns false, or when it is no longer among those the player may see, either unless its pvsflags has bit 128 (PVSF_NOREMOVE); one the player has whose bits are unchanged is passed over. Last SendFlags is cleared (SV_CleanupEnts) up to the highest entity that had some, and an entity whose Version differs from the count the server keeps for it (QuakeServer.Edicts.free/2 counts it up) gets SendFlags -1. Nothing is kept until a player watches.

The player may see an entity (SV_Snapshot_BuildQ1) when it is live, its customizeentityforclient (run with self it and other the player) returns true where it has one, and:

• it is the player or the player's view2, or its viewmodelforclient is the player; otherwise, with no viewmodelforclient, its effects have EF_NODEPTHTEST (8192), its pvsflags low bits are 3 (PVSF_IGNOREPVS), or 2 (PVSF_USEPHS) except on a Quake map with a PHS built (QuakeServer.Multicast.phs?/1), where its origin must be where the player's origin can be heard (QuakeServer.Visibility.heard/2); or else it touches a leaf of the player's fat PVS (QuakeServer.Visibility.fat/2 at the player's eye and at its view2), the entity at the end of its tag_entity chain (up to ten long) standing in for it, which passes as the player and fails as another player's viewmodelforclient;
• its nodrawtoclient is not the player, and its drawonlytoclient is none or the player;
• the player's dimension_see shares a bit with its dimension_seen or dimension_ghost, unless it is the player or the player's view2. Undeclared, dimension_see and dimension_seen read as 255.