Specifies valid field length (i.e., the number of characters) and valid characters for this data element.
Abbreviations: A=alpha (A-Z), N=numeric (0-9), S=special, F=fixed length, V=variable length.
Example: "V6ANS", "F3A", "F1N".
Target Platforms: .NET9; .NET 8; .NET 7; .NET 6; .NET Framework 4.8, 4.7, 4.6, 4.5, 4.0, 3.5