The heart of a decoder is the set of FIELD, GROUP, and associated statements that take values from given protocol fields and displays them in the decode window. Sometimes you want to directly display the value in a field in the form of a number; other times you want to display a string indicating what that value represents. Almost every protocol has fields that can be displayed most informatively by selecting a string from a table indexed by the field value.
Syntax rules and similar guidelines are offset from the main text of the tutorial in boxes like this one. Generally, such rules cover only simple cases of use.
Let's deconstruct these Field Statements.