Finally a bit closer to understanding the R13-300 vs R13F-300 engine differences, especially the evolution and principles of work of the "second stage afterburner or the ЧР" mode, bcs we can read very detailed R25-300 engine manual posted here.....
http://www.lovrichfamily.com/abgpdf/VTUP%20-%20Opis%20motora%20MM-17%20za%20avion%20L-17.pdf.......so far the engine manual is revealing the additional afterburner fuel pump "насос регулятор" NR-44 helping the primary afterburner fuel pump NR-22F2M2 almost to double the fuel flow when the "Чрезвычайний форсаж" or the co-called "second stage afterburner" was switched on. It provided a temporary increase(3 min) in thrust from 6850p(67,1kN) to 7100kp(69,6kN) during take-offs, but monstrous 9900kp(97kN) when flying up to 4000m altitude and speeds close to Mach 0,9. The "Чрезвычайний форсаж" on the R25-300 was supposed to be automatically disengaged when entering supersonic speeds.
The point here is that the predecessor R13-300 engine installed with the export Migs-21MF, despite of missing "ЧР" switch in the cockpit had a similar "second stage afterburner" mode, but only automatically triggered when the engine throttle was set to the far-end position (engaged first afterburning mode ) and the aircraft was flying at high altitude speed equaling Mach1.5. Then the engine control automatic increased LP compressor RPM to 103%, set engine nozzle diameter to max and the primary afterburning fuel pump NR-22F2 feeding fuel to the aferburning rings increased fuel flow via the DCN-44 aircraft fuel system pump, but which, compared to the NR-44 (R25-300) had very low efficiency, therefore thrust increase could be achieved only at supersonic Mach speeds (high engine compression ratios). The Mig-21MF(SM) with the R13-300 engines were the first Migs to use the "second stage afterburner", while the control light "Vtoraja Forsazh" lit on the T10U2 panel in the cockpit
The Mig-21SMT/MT/ST aircrafts had been equipped with the modified R13F-300(izd.95F) engine with the same kind of "ЧР" switch in front of the engine throttle lever as the R25-300 engine, which surely activated similar additional afterburner fuel pump during take-offs to help heavier SMT to get airborne and during maneuvering at high subsonic speeds providing afterburning thrust 8500kp(83kN) compared to 6600kp(64,7kN) on the R13-300 engine.