This property takes a comma seperated list of the valid types of principal it will return in searches etc. which relates to the names of the different values in the AccountType enumeration (See [http://msdn2.microsoft.com/en-us/library/microsoft.sharepoint.webcontrols.peopleeditor.accounttype.aspx]) - generally useful to get behaviour that allows only selection of groups i.e. by setting the value to "SPGroup".
- User = single user
- DL = AD distribution list
- SecGroup = AD security group
- SPGroup = Sharepoint group