Using a reserved word or symbol for a field or table name
FileMaker Pro reserves the use of some words and symbols, including:
Predefined parameters of some functions such as the Roman and Greek font scripts for the TextFont function.
Avoid using these words and symbols in field names and table names, because the names become difficult to refer to in calculations.
If one of your field names or table names is a reserved word or contains reserved symbol, you must put the characters ${ } around the name when it appears in a function.
Tip  When you double-click to choose a field for a calculation, FileMaker Pro automatically wraps the characters ${ } around field names that are reserved words or that contain reserved symbols.
${A + B} returns the contents of a field named A + B.
${.123} returns the contents of a field named .123.
${Pi} returns the contents of a field named Pi.
Note  During file conversion, FileMaker Pro uses the characters ${ } to enclose field names that conflict with reserved words and symbols.
Related topics 
About formulas
Defining calculation fields
Text operators
Using operators in formulas
Functions reference (category list)
Functions reference (alphabetical list)