In Salesforce Apex, when you need to define a method that can be called from Visual Flow, there are specific requirements and characteristics that the method should fulfill, especially when using the @InvocableMethod annotation. These requirements include:

Static Method: The method must be static because it is not associated with any specific object instance.
Handling Collections: The method should be able to process and return collections (i.e., lists) because invocable methods are designed to operate in bulk to handle multiple records at once. This is especially true when dealing with flows that can process multiple records.
Annotations Correctness: The annotation should be properly formatted, including the correct use of cases and syntax for properties like label.