 "What does compression style and 1/4 turn style (angle stop or straight stop) mean?"
 The compression style in this case is referring to the way the water is shut off when the valve is closed. Please do not confuse this with the inlet or outlet size which can also be referred to as "compression". The handle is attached to a stem with a rubber washer on the other end. As the handle is turned to close the valve (clockwise), the rubber washer is forced (compressed) against the "seat" which is the metal opening of the waterway. 1/4 turn style valves have inside the stem a port that when the handle is turned one quarter of a turn counterclockwise, the valve is open fully and the same quarter turn in the opposite direction closes the valve completely.
"How do I know if I need 1/2" Fip inlet or 5/8" o.d. inlet angle stops?"
Angle or straight stops have an inlet that is either 1/2" fip (female iron pipe) or 5/8" o.d. copper compression. The 1/2" fip stops are for attaching to threaded pipe nipples. The copper compression fittings are designed to attach to the smooth outside surface of copper pipe. 5/8" o.d. compression would fit a 1/2" nominal piece of copper.
"What is the difference between 'nominal' dimensions and 'outside diameter (OD)' dimensions?"
Nominal is the term used to describe most pipe and fitting measurements in the plumbing industry. Nominal refers to the inside diameter measurement of the pipe. The actual inside pipe measurements will vary slightly due to the wall thickness of the pipe, so it is referred to as "nominal" meaning a small variance of size. The outside diameter (OD) dimensions refer to the actual outside diameter of the pipe. The outside diameter dimensions of pipe are a constant size which allow pipe fittings to be used with the same type of piping with multiple grades of wall thickness.
"Some of the outlet sizes are shown as 3/8" O.D. or 5/8" O.D. compression nut w/sleeve. Is this measurement referring to the outside diameter of the compression nut?"
No, it is not. Copper compression fitting sizes are measured and sized according to the outside diameter (O.D.) of the copper tubing or copper pipe for which they will be used.
"What is CA AB 1953 and VT S.152?"
CA AB 1953 is a California Assembly Bill that defines "lead free" to mean not more than a weighted average of 0.25% lead content in pipe and fittings. VT S.152 is a Vermont Senate Bill that mimics the California AB 1953 and regulates the amount of lead to a weighted average of 0.25% or less.
"How does lead get into drinking water right now?"
Primarily, trace amounts of lead get into drinking water due to aging infrastructures, such as old pipes and plumbing systems parts. The Plumbing Manufacturer's Institute (PMI) states that over the past decade, lead levels in plumbing fixture fittings have been reduced to insignificant levels due to improvements in modern manufacturing processes.
"How are lead levels in drinking water currently regulated?"
The United States Environmental Protection Agency (EPA) regulates the amount of lead in drinking water under guidelines established in the Federal Safe Drinking Water Act (passed in 1974; amended in 1988 and 1996). This Act defines "lead-free" as not more than 8% lead in pipes and fixture fittings. PLEASE NOTE: Use of the term "lead-free" varies between the new legislation in CA (AB 1953) and VT (S.152), and in the current Federal Safe Drinking Water Act requirements. In the new legislation for CA and VT only, starting in 2010, it will mean 0.25% or less (weighted average).
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