The composeMail front-call started off when Genero Mobile was introduced as a mobile only front-call. It could be used by Genero Mobile developers to invoke the mobile devices default mail application and create a new mail populated with details such as To, Subject, Body etc.
In GBC 4.01.22, an equivalent composeMail front-call was added to the standard front-call library for use with the Universal Rendering interface that utilised the mailto URI scheme.
Read more at
https://4js.com/ask-reuben/ig-269/