I use them as shorthand for marking the real and imaginary parts of
numbers, since sometimes ℜ (U211C) and ℑ (U2111) are a bit much.
<Multi_key> <e> <m> <f> : "ℰ" U2130 # SCRIPT CAPIPAL E (electromotive force)
<Multi_key> <minus> <h> : "ℏ" U2105 # PLANC CONSTANT OVER TWO PI
+<Multi_key> <underscore> <i> : "ᵢ" U1D62 # LATIN SUBSCRIPT SMALL LETTER I
+<Multi_key> <underscore> <r> : "ᵣ" U1D63 # LATIN SUBSCRIPT SMALL LETTER R
+
"ff" UFB00 # LATIN SMALL LIGATURE FF
"fi" UFB01 # LATIN SMALL LIGATURE FI
"fl" UFB02 # LATIN SMALL LIGATURE FL