[Rd] Depends vs Imports

[Paul Gilbert]

I am being asked to modernize the Depends line in the DESCRIPTION file 
of some packages. Writing R Extensions says:

   The general rules are

      Packages whose namespace only is needed to load the package using
    library(pkgname) must be listed in the ‘Imports’ field and not in
    the ‘Depends’ field. Packages listed in imports or importFrom
    directives in the NAMESPACE file should almost always be in
    ‘Imports’ and not ‘Depends’.

      Packages that need to be attached to successfully load the
     package using library(pkgname) must be listed in the
     ‘Depends’ field, only.

Could someone please explain a few points I thought I understood but 
obviously do not, or point to where these are explained:

    -What does it mean for the namespace only to be needed? I thought 
the namespace was needed if the package or some of its functions were 
mentioned in the NAMESPACE file, and that only the namespace was needed 
if only the generics were called, and not other functions. The above 
suggests that I may be wrong about this. If so, that is, Imports will 
usually suffice, then when would Depends ever be needed when a package 
is mentioned in the NAMESPACE file?

   -Should the package DESCRIPTION make any accommodation for the 
situation where users will probably need to directly call functions in 
the imported package, even though the package itself does not?

    -What does "need to be attached" mean? Is there a distinction 
between a package being attached and a namespace being attached.

    -Does "successfully load" mean something different from actually 
using the package? That is, can we assume that if the package loads then 
all the functions to run things will actually be found?

    -If pkg1 uses a function foo in pkg3 indirectly, by a call to a 
function in  pkg2 which then uses foo, how should pkg1 indicate the 
relationship with foo's pkg3, or is there no need to indicate any 
relationship with pkg3 because that is all looked after by pkg2?

Thanks,
Paul