Returns a list of the names of all fields on layoutName, in fileName file, separated by carriage returns.
fileName - the name of an open database file (local or remote).
layoutName - the name of a layout or table in the specified database file.
Important  See Design functions for information about literal text parameters.
Data type returned 
Originated in 
FileMaker Pro 6.0 or earlier
Related fields are displayed in tablename::fieldname format.
If no parameter is specified for fileName, FileMaker returns results for the current file.
Note  If FieldNames returns a question mark (?) or the name of only one field, go to the Specify Calculation dialog box and make sure Calculation result is text. Also, you can increase the size of the field on the layout to show more field names.
FieldNames(“Customers”;””) returns a list of all the fields in the default table of the Customers database file.
FieldNames(“Customers”;“Data Entry”) returns a list of all the fields, including related fields, in the Customers database file that appear on the Data Entry layout.
Related topics 
Functions reference (category list)
Functions reference (alphabetical list)
About formulas
About functions
Defining calculation fields
Using operators in formulas