When you use the Customise button in RLC, it shows your avatar with what you're wearing at the time. You can change this in the Customise window. When you click "Show Changes" it saves the clothes, face, shoes et cetera you've chosen into a file that's probably in C:\Program Files\RedLightCenter\RedlightCenter\ and is called myinfo.ini, and it also changes what the rest of the users see you wearing.
If you click "Don't Change" it will instead
read this file to find out what you were presumably wearing before you clicked Customise, and will dress you in that. This does not change what everyone else sees because it is (presumably) what they were seeing anyway.
If you do something to that myinfo.ini file, save it and then go to Customise and click "Don't Change" it will load the new data onto your avatar as you see it, but noone else will see any changes. If you
then go back to Customise and click "Show Changes" it will show everyone else what you've just put on by loading the modified myinfo.ini file.
This means that you can choose your face, hair, clothes, shoes and so on and use "Show Changes" to save that as your appearance, and then copy the myinfo.ini file to, perhaps, myinfo_punkrocker.ini or myinfo_sporty.ini or whatever and thus get several outfits saved up. To put one on, copy it to myinfo.ini and then customise => don't change => customise => show changes as described above.
You can also manually edit myinfo.ini but
copy a working version of it before you poke it with a spoon! It's how the designers in
the textures forum test their stuff out, and it allows for wearing combinations you just can't get with the Customise button. Of particular interest is the
How to wear custom-made clothing thread, and there's some stuff about mixing up shoe textures, wearing underwear on the outside and so on in
my thread there.