"IF" is an initial fix, direct to which ATC can send a RNAV-equipped aircraft. Said aircraft can then intercept and fly the ILS, but the intercept angle cannot exceed 90 degrees.
If you are non-radar or NORDO, you have no other choice but to proceed from the enroute structure at a fix on the procedure to an IAF, and fly the entire procedure from the IAF, including any course reversals. Radar vectors are intended to put the aircraft on an INTERMEDIATE segment at an altitude compatible with the "platform" altitude i.e. a descent gradient of about 200 ft/nm. The intermediate segment begins with the course reversal. It has a low descent gradient due to the fact it is here the pilot is supposed to be slowing and configuring. The final segment begins at the FAF or FAP.
These "RADAR REQUIRED" approaches, as at DFW, have been controversial in the TERPS community as lacking a defined intermediate segment, prior to the appearance of an "IF" and RNAV. So, technically, when radar vectored, the pilot is vectored to a segment that does not exist.
GF