Historically, there are two types of PCB elements: file element
and m4 element. The former are files containing all the info
required to represent fixed footprints of the components corresponding
to them. The latter are files that should be processed by
m4 command to obtain the footprint data. An m4
element may generate several footprint definitions. Those m4
elements are supported only by the program pcb and it is
not recommended to use them as all m4 elements in pcb
have corresponding file elements. However, the m4
elements are still supported by lepton-sch2pcb. To deal with them, the
program uses lepton-netlist which, in turn, needs to know the name of a
directory of the pcb package containing the file
common.m4.