The following describes how spelling lists work under the TeX-mode.

An unhashed spelling list is a file (.usl file) containing words assumed to be
correct.  Each word should be placed in a single line.
A hashed spelling list is a file (.hsl file) which contains a hashed version of
the words listed in the .usl file.

There are two levels of spelling lists:
	(1) Per user spelling lists -- put the following in tex-mode-hook:
		(setq tex-usl-default <file-name>)
		(setq tex-hsl-default <file-name>)
	(2) Per document spelling lists:
 		foo.usl and foo.hsl will be created at the end of a
		spelling correction session upon request for a document
		rooted at foo.tex.
		Note that only the .usl file propagates through network.
		That means your .hsl on VAX will not work on SUN, but
		rcping the .usl file over will suffice.

The algorithm that determines which file to use as the spelling list is
as follows.  For a document rooted at foo.tex,
	(1) if foo.hsl exists and is newer than foo.usl, foo.hsl is used
	    as the spelling list.
	(2) if foo.hsl doesn't exist and tex-hsl-default is non-nil,
	    the file bound to tex-hsl-default is used.
	(3) otherwise, if foo.usl exists, a new foo.hsl is created,
	    by adding words in the file bound to tex-usl-default, if any,
	    and those in foo.usl to the system hashed list
	    (/usr/dict/hlista).
	(4) otherwise, no customized spelling list is used.

At the end of a spelling correction sesssion, the RET/s option
enters an incremental spelling list saving facility.
First, you'll be asked to reexamine uncorrected words, you can remove
whatever not supposed to go into the list.
Then, the incremental saving takes place.
The algorithm is as follows.
	(1) If foo.hsl exists and is newer than foo.usl, uncorrected words
	    from this buffer are added to the hashed list.
	(2) If foo.usl exists, uncorrected words are merged into the file;
	    otherwise a new file foo.usl is created to hold these words.
	(3) If foo.hsl doesn't exist, a new foo.hsl is created by
	    adding words in the new foo.usl and those in the file bound to
	    tex-usl-default, if any, to the system hashed list
	    (/usr/dict/hlista).