I looked at NASDAQ's List of Fifth Character Symbol Suffixes for NASDAQ tickers and FINRA's Fifth Character Indentifiers for OTC tickers. Both lists are similar, with some small differences.
My question is: are these fifth-character suffixes compulsory, or are they voluntary? For example, if I am looking at an arbitrary ticker, can I assume that it is an ADR if and only if the ticker is a five-letter ticker ending in "Y"? Similarly, if I am looking at an arbitrary ticker, can I assume that it is a warrant if and only if the ticker is a five-letter ticker ending in "W"?