Here is another new feature related to property data, coming your way in the soon to be released ADT 2007. You can now specify a Unit Type when setting up a Property Data Format, so that ADT "knows" what type of data to expect for that property. Please note that the Unit Types ONLY apply to manual properties, and only those set to the "Real" type. Auto Increment - Character, Auto Increment - Integer, Integer, Text and True/False types of manual properties do not make use of this new feature. You set the Unit Type in the Real Numbers area of the Formatting tab when editing the Property Data Format, as seen in the image below. The image also displays the list of different Unit Types available.
Each Unit Type then has an associated list of Units relevant to that type. You set a "default" Unit value in the PDF, and this will be used when first assigning a PDF to a new property. Once the property has a value assigned, that value "sticks" when changing to a new PDF, rather than changing to the default value of the new PDF, if the Unit Type is the same for both old and new PDFs. The Units available for the Length Unit Type can be seen in the image below.
Once you assign a PDF to a Real manual property, you can change the Units to any of those available for the assigned Unit Type in the Units column on the Definition tab, when editing a Property Set Defintion. Simply click on the current value to activate a dropdown list of available values. I will repeat that this only applies to Real manual properties; formula properties will be blank in the Units column, and the Automatic properties will list "dimensionless" but not offer any other choices in a dropdown list, as shown in the image below.
The final image shows the Extended Data tab for a wall that has the Property Set shown above attached. MyProperty and MyRealNumber both have the Length - Long Property Data Format attached, which uses the Architectural Unit Format and the Length Unit Type. MyProperty uses the default inches for the Unit, while MyRealNumber was changed to feet. The value typed into MyProperty was 38.75; the value typed into MyRealNumber was 12. These display as 3'-2 3/4" and 12'-0", respectively.
Note that if you want to use a number for mathematical purposes in a formula, you will want to stay away from the Architectural and Engineering Unit Formats, which convert numbers to imperial feet and inches format, which will be interpreted as a string, as well as avoiding text prefixes or suffixes. But you CAN use the Decimal Unit Format and assign a Unit Type and Units, such as Length and Inches, without creating a string. You can also choose to set the Unit Type to "(none)", as the out-of-the-box Standard Property Data Format does. This shows in the Units column of a Property Set Defintion as "dimensionless".
Also note that the Unit Type/Units settings affect the way the numbers input are interpreted, but not necessarily the way they are displayed. If your drawing units are inches, but you set up a Real manual property, in Decimal Unit Format, with units of Feet, when you enter the number, you are entering the value in feet, but after you hit return, the value will be displayed in inches [and the value will be twelve times bigger than you entered, since there are twelve inches in a foot]. Unless you have a specific case where you want to enter metric values, but display them in feet and inches, I would not recommend mixing the Architectural or Engineering Unit Format with a Length Unit Type and one of the metric units. I tried that, using Architectural with millimeters, and an entry of 1000 [mm] was displayed as 3'-3 3/8".