Forces Acrobat to search for the credentials on the specified server. If this option is not selected, the server is only suggested; Acrobat can use other available credentials if that the server is not available. If the server is available, it must use credentials even if they are not required. If you do not require a specific roaming credential server, Acrobat uses any digital ID regardless of its location.
See also 

Roaming ID tab (Signature Settings/Sign Data and Submit Settings dialog box)