What is the test of whether an object is a flag within the meaning of rule 61.1(a)?

In the context of rule 61.1(a), a flag is used as a signal to communicate the message “I intend to protest.“ Only if the object used as a flag communicates that message, with little or no possibility of causing confusion on the part of those on competing boats, will the object qualify as a flag. A flag must be seen primarily to be a flag.

USA 1988/277