An add-in function and its online help page are linked via a numeric Help ID, that is unique within the help file.
These IDs are generated (i) when the help file is built and (ii) at run-time when the XLL is loaded. Usually, the two sets of IDs will match, and no further effort is required to make the function connect to the appropriate help topic.
However, if your add-in uses a filter to select which functions will be registered (as in the PartialReg sample), or if you register add-in functions explicitly at run-time, then you will need to ensure that the Help IDs continue to match by setting a specific (and unique) Help ID for each add-in function.
You can do this for each function in the Properties window, by setting the Help ID property. (Note that a Help ID of 0 is considered to be unassigned.)
A quicker and easier way is to use the Set Help IDs command, which is on the Tools menu of the XLL Add-ins window. This command will create and assign a unique Help ID for all functions in the project which do not currently have a Help ID.
As a result, all of your add-in functions will be permanently tied to their corresponding help topic.