It's not hard to check, the guidelines are posted online: https://www.stroustrup.com/JSF-AV-rules.pdf#page17 https://www.stroustrup.com/JSF-AV-rules.pdf#page17

It's not hard to check, the guidelines are posted online: https://www.stroustrup.com/JSF-AV-rules.pdf#page17 https://www.stroustrup.com/JSF-AV-rules.pdf#page17 AV Rule 14 is the one in question. Uppercase suffixes are fine, lowercase is not, and the rationale is "Readability."