Webel: Mathematica: CONVENTION: "pattern helpers" (which are not themselves Patterns) use the prefix '$pat$. Encapsulated Patterns use a 'pat$' prefix, and may use a '$pat$' pattern helper (such as an Alternatives).

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Note kind
CONVENTION
NAMING
PATTERN
POLICY
Policy level
STRICT
Mathematica keywords
Pattern
Pattern template
Webel Mathematica keywords
'$pat$'
'pat$'
Keywords
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This naming pattern for Webel pattern helper for Mathematica is used throughout all Webel IT Australia coding of Mathematica libraries.

A trivial example: 
$pat$bool = (True|False)
Note that the pattern helper is not itself a Pattern. A Webel pattern encapsulator uses a pattern helper to create a Pattern (optionally with a specific named arg): 
pat$bool: myBool: $pat$bool;
Using a pattern encapsulator bound to a specific named argument is NOT IDE-friendly!
The '$pat$' convention is often used together with these Webel help holder conventions:
Webel: Mathematica: CONVENTION: Argument help holders use a String variable with the prefix '$arg$', a related '$info$arg$' String, a 'def$arg$' default value expression and/or 'req$arg$', and optional '$warn$arg$', '$tip$arg$, '$lab$arg$', 'type$arg$'..
Webel: Mathematica: CONVENTION: Option help holders use a String variable with the prefix '$opt$', a related '$info$opt$' String, a 'def$opt$' default value expression, and optional '$warn$opt$', '$tip$opt$', '$lab$opt$', and 'type$opt$' indicator ...
There is a detailed technical slide trail describing how the Webel naming conventions for Wolfram Language code work together:
Visit also: SECTION: Webel coding and naming conventions for the Wolfram Language
Relates to
The Webel ‘$’ prefix convention for Strings and "String similar" Wolfram Language code
The Webel ‘$pat$’ pattern helper convention for frequently used Patterns
Related notes
[POLICY]{STRONG} About the Single Source of Truth (SSOT) and Don't Repeat Yourself (DRY) principles
[CONVENTION, NAMING, PATTERN, POLICY]{STRICT} Webel: Mathematica: CONVENTION: A function's description (but not its signature, arguments, or options) may be captured as an '$info$f$' Stringhelp holder for use by the Doc` package for ::usage generation and the HelpF`& HelpO` help registry packages.
[NAMING, PATTERN, POLICY]{STRICT} Webel: Mathematica: CONVENTION: Argument help holders use a String variable with the prefix '$arg$', a related '$info$arg$' String, a 'def$arg$' default value expression and/or 'req$arg$', and optional '$warn$arg$', '$tip$arg$, '$lab$arg$', 'type$arg$'..
[NAMING, PATTERN, POLICY]{STRICT} Webel: Mathematica: CONVENTION: Option help holders use a String variable with the prefix '$opt$', a related '$info$opt$' String, a 'def$opt$' default value expression, and optional '$warn$opt$', '$tip$opt$', '$lab$opt$', and 'type$opt$' indicator ...
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SECTION: Webel coding and naming conventions for the Wolfram Language
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