Question about native vs regular search paths

[(]

Hi,

What is the difference, in concrete terms, between SEARCH-PATHS and
NATIVE-SEARCH-PATHS; why do they need to be separate, why is
SEARCH-PATHS seemingly only used during a build, and why are
NATIVE-SEARCH-PATHS only set in cross-compiled builds?

I'm asking because I want to understand why <BAG> has TARGET-INPUTS:

--- guix/build-system.scm
  ;; "Target inputs" are packages that are built natively, but that are used
  ;; by target programs in a cross-compilation environment.  Thus, they act
  ;; like 'inputs' as far as search paths are concerned.  The only example of
  ;; that is the cross-libc: it is an input of 'cross-gcc', thus built
  ;; natively; yet, we want it to be considered as a target input for the
  ;; purposes of $CPATH, $LIBRARY_PATH, etc.
  (target-inputs bag-target-inputs
                 (default '()))
---

This comment explains what happens, but I don't get *why* this has to
happen; what's the reason cross-libc has to be built natively, and why
does it need to be treated as a target input for SEARCH-PATHS?  What
does 'being considered as a target input' even mean in that context?

Thanks,

  -- (