Quick Explanation

When creating a custom node the end use of your graph is to make something that you will use in another substance usually to either speed up your work or to give you more options/control when working.

For example you may have a warp blur combo that has a user input going into a warp and then the warp going into a blur and then into an output. This could be a simple set up you use often but you choose to make it something you can either search for (by putting into the program files) or drag into a graph you are working on. Then the graph would act like a standard node that you are used to. You can give more or less control of these custom nodes by exposing parameters much the same as you would with a material or anything else.

When exporting out .sbsar files with inputs ensure to set them to absolute and the the setting you want because

Once cooked, the output format is static. If you need a 8 bits and 16 bits version, you have to publish your graph twice.