A term, indicated by a superscript ', which means that a process is not well known or that its value carries uncertainties which are not known, e.g. \(\Delta G^{\,\unicode{x26ac}}′\) is the apparent standard Gibbs energy change. In the context of @P04418@ the word is used in a different sense; as a symbol for 'apparent' in this connection the use of subscript \(\varphi \), as in \(Y_{\varphi }\), is recommended. Other notations employed for this property include \(^{\varphi }X\) and \(\varphi _{X}\).