AMSA84 - it is simply the difference between (a) STATIC values (DC currents and voltages, capital letters ) and (b) differential (dynamic) values (small ac values, small letters).
Examples:
* An (idealized) ohmic resistor has a linear I-V characteristic. Hence, there is no difference between R1 and r1
* Diodes, transistors,.. have non-linear current-voltage relationships. Hence, the SLOPE of the characteristic depends on the bias conditions (DC operating point) and , thus, determines the differential values (written in small letters).
* For my opinion, the transconductance of such a device (BJT,FET) should always be written with small letters (gm) because it always is a differential quantity.
It is rather important to distinguish between static and dynamic values - otherwise one cannot understand the meaning of some quantities.
Example: In some books, instead of the transconductance gm its inverse value is used ro=1/gm (in some cases it is even written as Ro). I think, such a terminology is not correct because this quantity is not a resistive value because it connects an output current with an input voltage.