.encfol files
These are just VeraCrypt container files.

.encopt files (optional)
Encrypted folder option files.  When one such file is present next to an .encfol file, it's content will be read and all options present in it presented to the user.  Each line may either be an option section name, a "run" command line, a "shellexecute" command, or an empty line.  The option to apply will be asked for when mounting the container.

Section names are formatted like "[Section name]".  Each section can contain more than one command line; they will be ran in sequence.  The command lines must be formatted as <type letter>=<command>, where <type letter> is either "R" or "S".  The command lines may contain the "[MOUNTPATH]" tag, which will be replaced with the actual mount path used.

When no .encopt file is specified, the command <S:"[MOUNTPATH]"> is used, which results in the mounted folder opening in Explorer.