If, for some reason, you want to spread your PLUMED input over a number of files you can use INCLUDE as shown below:
INCLUDE FILE=filename
So, for example, a single "plumed.dat" file:
DISTANCE ATOMS=1,2 LABEL=dist RESTRAINT ARG=dist AT=2.0 KAPPA=1.0
could be split up into two files as shown below:
DISTANCE ATOMS=1,2 LABEL=dist INCLUDE FILE=toBeIncluded.inc
plus a "toBeIncluded.inc" file
#SETTINGS FILENAME=toBeIncluded.inc # this is toBeIncluded.inc RESTRAINT ARG=dist AT=2.0 KAPPA=1.0
However, when you do this it is important to recognize that INCLUDE is a real directive that is only resolved after all the Comments have been stripped and the Continuation lines have been unrolled. This means it is not possible to do things like:
# this is wrong: DISTANCE INCLUDE FILE=options.dat RESTRAINT ARG=dist AT=2.0 KAPPA=1.0