They're elementary in NYC

• They may follow signatures: Abbr
• Letter addenda: Abbr.
• Remarks akin to "btw"
• They follow closings: Abbr.
• Additions to ltrs
• Abbrevs. that signify "OH SHIT I forgot to write this in the letter..."
• Letter addenda (abbr.)
• Often-overlooked email parts, briefly
• They go below signatures, for short

Wikipedia Word definitions in Wikipedia
PSS may refer to: