Unicode has a region (0xe000-0xf8ff) in which font vendors may assign their own characters. Adobe and Apple have each done so and FontForge recognizes the most common of these (things like the Small Caps characters used in Adobe's Expert Character Set, or the Apple Logo used in nearly all Apple fonts). The allocations made by these two companies are described in:
I have been asked to add a few similar characters to FontForge's repertory and so have started using my own block of characters 0xf500-0xf580.
f500;Alphasmall;GREEK SMALL CAPITAL LETTER ALPHA;<sc>;0391 f501;Betasmall;GREEK SMALL CAPITAL LETTER BETA;<sc>;0392 f502;Gammasmall;GREEK SMALL CAPITAL LETTER GAMMA;<sc>;0393 f503;Deltasmall;GREEK SMALL CAPITAL LETTER DELTA;<sc>;0394 f504;Epsilonsmall;GREEK SMALL CAPITAL LETTER EPSILON;<sc>;0395 f505;Zetasmall;GREEK SMALL CAPITAL LETTER ZETA;<sc>;0396 f506;Etasmall;GREEK SMALL CAPITAL LETTER ETA;<sc>;0397 f507;Thetasmall;GREEK SMALL CAPITAL LETTER THETA;<sc>;0398 f508;Iotasmall;GREEK SMALL CAPITAL LETTER IOTA;<sc>;0399 f509;Kappasmall;GREEK SMALL CAPITAL LETTER KAPPA;<sc>;039a f50a;Lambdasmall;GREEK SMALL CAPITAL LETTER LAMBDA;<sc>;039b f50b;Musmall;GREEK SMALL CAPITAL LETTER MU;<sc>;039c f50c;Nusmall;GREEK SMALL CAPITAL LETTER NU;<sc>;039d f50d;Xismall;GREEK SMALL CAPITAL LETTER XI;<sc>;039e f50e;Omicronsmall;GREEK SMALL CAPITAL LETTER OMICRON;<sc>;039f f50f;Pismall;GREEK SMALL CAPITAL LETTER ALPHA;<sc>;03a0 f510;Rhosmall;GREEK SMALL CAPITAL LETTER RHO;<sc>;03a1 f511;; f512;Sigmasmall;GREEK SMALL CAPITAL LETTER SIGMA;<sc>;03a3 f513;Tausmall;GREEK SMALL CAPITAL LETTER TAU;<sc>;03a4 f514;Upsilonsmall;GREEK SMALL CAPITAL LETTER UPSILON;<sc>;03a5 f515;Phismall;GREEK SMALL CAPITAL LETTER PHI;<sc>;03a6 f516;Chismall;GREEK SMALL CAPITAL LETTER CHI;<sc>;03a7 f517;Psismall;GREEK SMALL CAPITAL LETTER PSI;<sc>;03a8 f518;Omegasmall;GREEK SMALL CAPITAL LETTER OMEGA;<sc>;03a9 f519;Iotadieresissmall;GREEK SMALL CAPITAL LETTER IOTA WITH DIAERESIS;<sc>;03aa f51a;Upsilondieresissmall;GREEK SMALL CAPITAL LETTER UPSILON WITH DIAERESIS;<sc>;03ab
f520;c_t;LATIN SMALL LIGATURE CT;;;0063 0074 f521;longs_i;LATIN SMALL LIGATURE LONGS I;;;017f 0069 f522;longs_l;LATIN SMALL LIGATURE LONGS L;;;017f 006c f523;longs_longs;LATIN SMALL LIGATURE LONGS LONGS;;;017f 017f f524;longs_longs_i;LATIN SMALL LIGATURE LONGS LONGS I;;;017f 0x17f 0069 f525;longs_longs_l;LATIN SMALL LIGATURE LONGS LONGS L;;;017f 0x17f 006c