It actually says 'call can invoke a proc defines below it.'
Like other rules around it (eg. 'ifeq, ifneq and wneq can be nested, can appear inside a proc')
They all use can instead of have to or must. Which means proc defined above is also acceptable (likewise, ifeq does not have to be nested or have to be inside a proc)

Please let me know how to improve it if it implies too much distracting infomation