Can anybody explain this uucp spool directory setup?

[Bill Swan]

In our directory (/usr/spool/uucp/) there are several subdirectories. I am
not entirely sure what they are used for, and the documentation for our 4.2bsd
system makes absolutely no mention of them. I have made some guesses as to the 
functions, but would appreciate correction (to aid a program I am writing).

In /usr/spool/uucp:	My best guess:		Wild guess:

	C./		command (work) files
	D.sigma/	outgoing "data" files
	D.sigmaX/	outgoing execute files
	D./					incoming "data" files?
	TM./		incoming ("data"?) files, temporary
	X./					incoming execute files?
	XTMP/					incoming (exec) files, temp?

Also, in the document _Uucp Implementation Description_ by D.A.Nowitz, pg 5,
there is a description of how "work-related file" names are generated. There
is mention of one character in the name described as "Grade", but no other
mention to that character nor what "Grade" is is made. Does anyone know?
-- 
William Swan  {ihnp4,decvax,allegra,...}!uw-beaver!tikal!sigma!bill