globalname
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globalname is a keyvalue available in all 
Source games. It's a name to designate two entities in different maps as being logically the same entity. When player transitions from map A to B the entity in map B with matching globalname will get most of its properties from entity that was in map A like origin, angles, color etc. (model for example is not transferred). If the entity was killed in map A it will not spawn in map B.
Warning:Setting this in round based games will cause an entity to not respawn on round restart because entities are killed when round ends.
Important:Don't use in 
Left 4 Dead series as the transition works differently and globalname is not used in this manner.
Placement Tip:This is important when wanting to transition brush entities as brush models are saved within a map file and is the reason model is one of the properties that isn't transferred.Other ways to ensure no duplicate (logically same) entities between maps transition is properly utilizing logic_auto outputs like OnNewGame, OnMapTransition.
Example
Note:There are several in 
Half-Life 2d1_canals_05 / d1_canals_06
To properly preserve states of the following entities
- func_breakables (on the left with globalnames 05.breakable02_1 to 05.breakable02_6)
- func_physboxes (on the right with globalnames 05.breakable01_1 and 05.breakable01_2)
After destroying the func_breakables then using dump_globals command we can see global states 05.breakable02_1 to 05.breakable02_6 marked as Dead which will ensure they will not spawn in next map.
] dump_globals is_pc: d1_canals_05 (On) = 217026304 -- Globals -- 05.breakable01_2: d1_canals_05 (On) = 12110476 skill.cfg: d1_canals_05 (On) = 0 05.breakable01_1: d1_canals_05 (On) = 12110476 is_console: d1_canals_05 (Off) = 1560839504 05.breakable02_4: d1_canals_05 (Dead) = 12110476 05.breakable02_1: d1_canals_05 (Dead) = 12110476 05.breakable02_5: d1_canals_05 (Dead) = 12110476 05.breakable02_6: d1_canals_05 (Dead) = 12110476 05.breakable02_2: d1_canals_05 (Dead) = 12110476 05.breakable02_3: d1_canals_05 (Dead) = 12110476
Changing one of the func_physboxes color to red using ent_fire !picker color "255 0 0" for the sake of demonstration.
After transition we can see states were preserved and model name being different (*112 and *113 in d5 changed to *162 and *161 in d6).
Rough entity transition logic
gGlobalState - object containing list of global states that persists between transitions, each element consists of "name, map_name, state (ON/OFF/DEAD), counter"
- Changelevel begins
- Remember all entities that are supposed to transition which means
- They are in info_landmark's PVS unless trigger_transition exists which is then used instead
- They are capable of transitioning which means they have proper ObjectCaps or globalname
- Next map loads, all entities spawn and following applies:
- Entities with globalname where global state doesn't exist yet normally spawn and are tracked by adding state
"name: <globalname>, map_name: <current_map>, state: ON"to gGlobalState - Entities with globalname where global state is ON and map_name is not current map are made dormant and invisible waiting to get their non-global properties overwritten by global entity that transitions
- Entities with globalname where global state is DEAD are not spawned
- Entities with globalname where global state doesn't exist yet normally spawn and are tracked by adding state
- Remembered transitioning entities spawn, following happens for each that had globalname:
- Search for already spawned entity (the dormant one) (if it doesn't exist print warning and leave the entity that were to transition in previous map)
- Mark self for removal
- Set the associated global state's map_name to current_map
- Transfer all non-global properties to the corresponding dormant entity (see datamap dump to see which are non global for example Half-Life 2 datamap dump)