When I make up an .ivu I have a "Source=" and a "FilePath="

I was editing an existing .ivu in Notepad I thought that Source would be the name of the input file and FilePath would be the path the file was in but when I entered this:

Source=Data.txt
FilePath=D:\Industry

the Builder (instead of looking at the file in my D:\Industry directory) found the file in the same directory as my .ivu was in.

What exactly do these 2 lines tell the Builder?

I can see why this is confusing.

The parameters used in the [Input] section of the IVU file can be somewhat misleading.

The "Source" parameter is required, it specifies the name of the source file.
The "FilePath" parameter is optional, and it specifies the name of the source files, when multiple files are listed as sources. Note that even though multiple files are listed as a source, one file still has to be defined as the "Source" parameter. The FilePath will accept wild cards like "?" or "*". As an example, "data???.txt" will specify that all file names that start by data, followed by three characters, and end with .txt will be valid source files (data001.txt, data002.txt, ..., datannn.txt).

Both parameters will follow the regular path logic, which is the following:
If no path is specified, it will first look in the working folder, then it would look into the different folders defined by the environment variable "PATH".
You can use a relative path from the working folder, like "..\Data\Data.txt" or "Text\Data.txt"
You can also use an absolute path, like "C:\Beyond2020\Data\Data.txt" or "C:\Beyond2020\Builder\Text\Data.txt"

So, in your example, it would look for "Data.txt" in the working folder, then in the folders defined by the "Path" variable. it would also ignore your FilePath parameter since no multiple source files were specified.

I guess you wanted to use D:\Industry\Data.txt as your source file, therefore, your IVU input section should have looked like this:

[Input]
Definition=IVPLUS.DLL
Source=D:\Industry\Data.txt