Hi Josh - Yes, we are hoping to have the final 4.1 user guide available to everyone in the next week but still working on a few of the details related to the guide.
Basically, this is a new feature that was introduced in 4.1 that can allow you to use certain HTML field type (within our module) and then add in standard HTML form field variables with the ID that includes DFCustom_ within the name/id of the field.
So...
For example... You can see standard fields here:
http://www.w3schools.com/html/html_forms.asp OR here:
http://www.echoecho.com/htmlforms04.htm
An example would be:
<code>
Last Name: <input type="text" name="DFCustom_lastname" />
</code>
That could then be referenced with the value of $(DFCustom_lastname) within email and SQL events etc...
Thanks,
Chad