Scope: managed applications, mobile applications, and ordinary applications.
In most cases, FormAttributeToValue is preferable to FormDataToValue.
Sticking to this recommendation will help you to keep the code consistent with other 1C:Enterprise applications. Also, FormAttributeToValue has simpler syntax, that is, less chance to make a code mistake.
SignaturesTable = FormDataToValue(SignaturesTable, Type("ValueTable"));
In contrast, FormAttributeToValue doesn't require data type to be specified. Example:
SignaturesTable = FormAttributeToValue("SignaturesTable");
1C:Enterprise supports both the FormAttributeToValue and FormDataToValue methods, but the last-mentioned is considered more usable. In terms of efficiency and output, the methods are equal.