ASTM E527 / SAE J1086  are the UNS guide specification.

There are no fast rules concerning anything other than the letter prefix and some broad alloy classes.  (P is for precious metals, P00001-P00999 is gold)

I belive that UNS assignment is being handled by a gentleman in Pittsburgh.  He has done a very good job at incorporating common designations into UNS numbers.
When you request that an alloy be given a number you are allowed to ask for a specific number, but SAE has the last word.

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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

For stainless, the first three (or four for alloy steels) are typicially the AISI/SAE designation, with the last two (or one) being use to denote a certain temper, or processing method, or a chemistry modification of the "standard" grade.  At the present time, there is no real convention on what the last numbers should be (at least not that I'm aware of).

In general (from MatWeb):

Overview of the UNS system

This is an overview of the UNS system, with special emphasis on common commercial alloys. As with any system, there are ambiguities such as the distinction between a nickel-based superalloy and a high-nickel stainless steel.

Axxxxx - Aluminum Alloys
Cxxxxx - Copper Alloys, including Brass and Bronze
Fxxxxx - Iron, including Ductile Irons and Cast Irons
Gxxxxx - Carbon and Alloy Steels
Hxxxxx - Steels - AISI H Steels
Jxxxxx - Steels - Cast
Kxxxxx - Steels, including Maraging, Stainless, HSLA, Iron-Base Superalloys
L5xxxx - Lead Alloys, including Babbit Alloys and Solders
M1xxxx - Magnesium Alloys
Nxxxxx - Nickel Alloys
Rxxxxx - Refractory Alloys
R03xxx- Molybdenum Alloys
R04xxx- Niobium (Columbium) Alloys
R05xxx- Tantalum Alloys
R3xxxx- Cobalt Alloys
R5xxxx- Titanium Alloys
R6xxxx- Zirconium Alloys
Sxxxxx - Stainless Steels, including Precipitation Hardening and Iron-Based Superalloys
Txxxxx - Tool Steels
Zxxxxx - Zinc Alloys

Most alloys that are reciveing UNS numbers these days do not have ANSI grade desinations.  Often the numbers being selected are related to the common chemistry notation, such as S32205 for 2205 duplex.
On the older alloys they did follow a pattern, 00 for hte straight grade, 03 for the L grade and so on, but not any more.

There are the 18 basic catagories (don't forget W for weld fillers).
E, L, M, P, R, and W are broken down further by type of alloy.

Buy a UNS book from ASTM.  It lists all current and deleated UNS numbers, spec cross references and the governing spec is printed in the back.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm