rfc/prefer_rfcdes is a SAP Parameter attribute which is used to control forces the use of the tables rfcdes contend instead of resolving the information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
For controlling the profile parameter
rfc/prefer_rfcdes
0
(default) the destinations of type <(><<)>HOST>_<(><<)>SYS>... are
expanded to <(><<)>HOST> and <(><<)>SYS>
1
The destinations of this type are not expanded directly but instead it
is searched in the table RFCDES for a suitable destination
See SAP Parameter Documentation for full SAP documentation for this profile parameter.