Strictly speaking, DDI/DKI software property names have no restrictions. Certain uses are recommended, however. The IEEE 1275-1994 Standard for Boot Firmware defines properties as follows:
A property is a human readable text string consisting of from 1 to 31 printable characters. Property names cannot contain upper case characters or the characters “/”, “\”, “:”, “[“, “]” and “@”. Property names beginning with the character “+” are reserved for use by future revisions of IEEE 1275-1994.
By convention, underscores are not used in property names. Use a hyphen (-) instead. By convention, property names ending with the question mark character (?) contain values that are strings, typically TRUE or FALSE, for example auto-boot?.
Predefined property names are listed in publications of the IEEE 1275 Working Group. See http://playground.sun.com/1275/ for information about how to obtain these publications. For a discussion of adding properties in driver configuration files, see the driver.conf(4) man page. The pm(9P) and pm-components(9P) man pages show how properties are used in power management. Read the sd(7D) man page as an example of how properties should be documented in device driver man pages.