Every column input in a form can prompt the user with a list of suggested values. For example, a column Industry of type Text could have 'Tech', 'Media' and 'Construction' as suggested values.
The list of suggested values is optionally defined by an expression in the column definition. The expression could be a constant (eg: LIST("Tech", "Media")) or a dynamic expression (eg: SELECT(LookupTable[Option], [Category] = [_THISROW].[Category]).
Please note that the Suggested Values constraint is a "soft" constraint. This list of values is not meant to be exhaustive or constraining. The user will be able to enter any other value that conforms to the column type (and the Valid_If constraint if one has been defined).