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From Odwiki

It's possible to define subnetworks of operations with any of its operations' parameters exposed to the custom operator. This presents the user with only the parameters of your choosing, effectively allowing you to build your own reusable toolset within Houdini. By default, these constituent operators are referenced off disk whenever you load the scene or opt to "refresh" your operators. This avoids redundant definitions of the contents on disk and it provides a nice way to ensure your nodes have synchronised functionality.

How to create a CustomSubnetOperator

The easiest way to create one is to collapse the system of nodes that you feel would benefit being a single node into an ordinary subnet. Now Ctrl+RMB on the node and select "Create Type From..." and this will pop up the Properties.. page for you to tell it where to save this definition. Now add any parameters to the subnet you want and edit the nodes inside the subnet to access these parameters.

Where can you create CustomSubnetOperators?

You can currently create CustomSubnetOperators in all Houdini contexts: Object, SOP,POP,COP,CHOP,VOP and OUTPUT but not in SHOPs. This means that if you want to integrate your VOP network on which your shop is based, you have to make a CustomSubnetOperator of say the object type and put a SHOP Manager in there accompanied by a VOP Manager, to make your shaders profit from the automatic OTL updates and still have your VOPS on which they are based, at hand.

Scripted Subnets - obsolete: pre-Houdini 6

A ScriptedSubnetOperator is a way to define custom operators for Houdini. This method was the predecessor of OTL and uses IndexFiles located in $HOUDINI_PATH/subnet/SUB??? and which point to hscript scripts usually located in $HOUDINI_PATH/scripts/{Type}.

To make such an operator, you would collapse an operator into a subnet, bring the parameters of the orignal node 1 level up using relative linking and then right click to "Create Subnet Operator". These days (Houdini 6.1.208), when you bring up "Type Properties" of such a subnetwork, houdini suggests saving it in an OPCustom.otl file, effectively upgrading this operator to an otl.