I need to learn how to read model number on all units. Example: What does the number HP20-411-1P tell me about a Lennox unit. Is there any web sites that list this information for units and compressor or other parts.

I have not found and do not expect to find a site that lists every manufacturer's model number ever made. This particular Lennox model [HP] Heat Pump of [411] 41,000 btuh (3-1/2 ton) [1P] single phase residential unit is easier to read as I was a Lennox Heat Pump contractor for many years. Many manufacturers' identifiers are more obscure; but the factory representatives can give you the identification.