To control whether a capture template should be accessible from
a specific context, you can customize
org-capture-templates-contexts. Let’s say, for example, that you
have a capture template “p” for storing Gnus emails containing
patches. Then you would configure this option like this:
(setq org-capture-templates-contexts
'(("p" ((in-mode . "message-mode")))))
You can also tell that the command key p should refer to another template. In that case, add this command key like this:
(setq org-capture-templates-contexts
'(("p" "q" ((in-mode . "message-mode")))))
See the docstring of the variable for more information.