The Lackey option provides Source Types based on fourteen models given in the older 1980 book while the Mills options provides over one hundred Source Types based on the 1997 book.  While there are similarities between the Source Types of the two Source Categories, the major difference is that the Mills Source Types tend to be specific to a single document while the Lackey Source Types tend to be more generic and can often apply to multiple similar documents (especially with minor adjustments).  

Users should select one or the other of the two Source Categories and base their Source Definitions on the Source Types of that Source Category for the sake of consistency.  But, it is believed that user can learn more about source citations in TMG from comparing the Source Types of Lackey with those of Mills.  This can be done by temporarily creating a special project and initializing the Source Category to Lackey if they already use Mills or vice versa.  However, some users may wish to have both Source Categories in the same project to make the comparison easier.  This can be done easily by exporting the Source Types from a project using the normally unused Source Category and then importing that file into a project with the normally used Source Category.  Exporting and importing is done by clicking on the [Export]/[Import] buttons in the Source Types window (Tools=>Source Types).

The downside of having both Lackey and Mills Source Types in the same project is that it is not easy to distinguish between the names of the Source Types.  For this reason, I have edited the normal names of the Source Types for the two Source Categories by prefixing each with an "L" (Lackey) or "M" (Mills).  Then I exported the Source Types into two files: Lackey and Mills which may be downloaded and imported to your project.

When you download either or both files, you should place it/them in the Export folder of the User Data Files folder (normally in the user's MyDocuments (WinXP) or Documents (WinVista) folder).  This makes the file(s) easily visible when you start importing although you can place the file(s) anywhere with a little browsing effort during import.  

After downloading the file(s), you must re-name it/them so that the file extension is changed from "zip" to "xst".

Note there are differences in the design style of Mills and Lackey.  As noted above, some of this difference allows for Lackey Source Types to more easily be adapted for multiple sources or for generic sources.   Another difference is in how the Bibliographies of some Mills Source Types sort compared to the corresponding Lackey Source Types.  This sort difference is mainly for unpublished Source Types although others are also affected.