Instance
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An instance is a map (.vmf) file referenced inside another map through a func_instance, possibly aided by a func_instance_parms and/or func_instance_io_proxy.
Note: In order for instances to be compiled into maps, a GameData key must be present in gameinfo.txt pointing to the game's FGD file. (To do: Is InstancePath needed as well?) Only 
Left 4 Dead 2 and later, along with 
Team Fortress 2, include this by default. Instances can be enabled in other games by adding this, or using a pre-compiler as described below. Note: In pre-Left 4 Dead 2 branches, instances have some broken or missing functionality: instance I/O system is unsupported, instances cannot be nested, displacements within an instance may not compile properly and any env_cubemaps inside of an instance will not be compiled into the final map.Instances are helpful in many ways:
- They provide a more dynamic alternative to prefabs, as any changes to the instanced map will be reflected in all instances of it.
- They provide an alternative to visgroups, as they can be used to divide a large, unwieldy map into several smaller, manageable ones (that can even be worked on by multiple authors simultaneously).
- They provide a simple way to edit portions of a map that needs to be at a non-orthogonal angle. (Build the map on-grid inside a separate map, and then instance it into another map through a func_instance rotated at the correct angle.
- They provide a way for maps to work on multiple platforms, such as Left 4 Dead and Left 4 Dead 2 versions of the same map.
Instance I/O
In Left 4 Dead 2 and later, instances can send and receive inputs and outputs. To use this functionality, a func_instance_io_proxy entity named proxy must be present and properly configured in the instance. To send an input to an entity within the instance, the following output would be used:
My Output Target Entity Target Input Parameter Delay Only Once 
<output> <instance> instance:<entity>;<input> <none> 0.00 No
To receive an output from an entity in the instance, this output would be used on the func_instance:
My Output Target Entity Target Input Parameter Delay Only Once instance:<entity>;<output> <other entity> <input> <none> 0.00 No
Note: Hammer should auto-complete these inputs and outputs. They may also appear as invalid, but will work fine in-game.In older branches
Pre-L4D2 engine branches do not support the instance I/O system, so workarounds are needed to send and receive inputs and outputs in instances. For inputs, the fixed up entity name can be referenced directly.
My Output Target Entity Target Input Parameter Delay Only Once <output> <instance>-<entity> <input> <none> 0.00 No
Note: The target entity name will vary depending on the Entity Name Fix Up set on the instance.Manifests
Instances can be taken a step further with manifests. A manifest splits all of the contents of a map into special instances known as "sub-maps". Unlike regular instances, Hammer can seamlessly switch between sub-maps without having to open the VMFs separately. Sub-maps are treated as extensions of the map itself and cannot use name fixup or have any offset, although sub-maps are capable of being used in multiple manifests simultaneously.
Manifests can be helpful in many ways:
- They allow multiple people to work on a map at the same time.
- They help protect a map against corruption, as problems with a VMF are limited to a single sub-map.
- They can be used to assign user-friendly names to different areas of a map.
- They can make a map's source files easier to track in version control software like Git.
A manifest can be created in the "Instancing" menu.
Warning: Manifests should be mostly functional in any[confirm] version of Hammer, but in most games, VBSP's support for manifests is unfinished and cannot compile them properly. VMMC can collapse manifests into a single VMF before compilation, which fixes many of these issues.
Code Fix: For 
Source 2013 mods, many of VBSP's issues can be fixed with code modifications. This fork by DeathByNukes includes several overall manifest fixes. Mapbase's repository modifies these fixes to work in 
Half-Life 2 mapping and also incorporates instancing fixes intended for manifests.Precompiler
Metapyziks wrote a program that merges instances into the main VMF before each compile. This was originally created to enable instance support in older engine branches, but the GameData method has since rendered it mostly obsolete. However, it would theoretically be possible for someone to fork the program to add additional functionality, such as instance I/O support or the ability to toggle VisGroups with a fixup variable.
Also see
- L4D2 Level Design/VMF Instances - A Valve tutorial on how to use instances in Left 4 Dead 2.
- Working with instances - A tutorial on how to use instances in Portal 2.
