Ultima Online File Format Document : Visual Basic .NET

Index

Index files are used to lookup data in other MUL files.

Data:

Dim lookup As Integer
Dim length As Integer
Dim extra As Integer

To find data for a certain entry, you seek to (EntryID * 12), and read the data.
Then, in the actual MUL file (not the index file), you seek to (lookup), and read (length) number of bytes.
Some data relies on the extra field.