You can define context menus of the Planning main tab in SHELL4_TABITEMS.XML file, GN4ShellConfiguration > GN4ShellMenus > Menuset Name="PlanningMainTab" > ContextMenu.

All context menus (by main tab)

Articles, Collections, Contacts, Editions, Media Galleries, Pages, Planning, Publishing, Reports, Shell, Start, Tracking, TrashCan

See also

Configuring menus