The system variable identifier is Sys_EnableErrorUnder, and the more readable name is Enable error underlining. This has possible values 0 (false) and 1 (true). When true, this applies to expression fields such as definitions in object windows or the attribute pane, and the underlining is intended to indicate errors that the Intellisense parser detects. These error indications are continuously updated as you type in expression fields. You can turn this off if the underlining is too distracting. Note that the Intellisense parser does not do full analysis, and so there are errors in definitions - such as unrecognized identifiers that are not the identifiers of any nodes in your model or names of any existing functions - that are not flagged as errors when you type. When you finally use the green checkmark or close the object window or attribute pane or attempt to evaluate a node complete checking will be done that will pinpoint errors not already flagged in a preliminary way during editing.
Note that syntax coloring will distinguish between identifiers that successfully refer and identifiers that are not resolvable. The latter are termed "variables" in the edit table for Sys SyntaxColors.
For information that might help putting this in context see Syntax coloring and fonts and error underlining.