Clarify the difference between variables and parameters

[odow]

Closes #2500

[codecov]

Codecov Report

Merging #2502 (0e0bff8) into master (684aed0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2502   +/-   ##
=======================================
  Coverage   92.42%   92.42%           
=======================================
  Files          43       43           
  Lines        4688     4688           
=======================================
  Hits         4333     4333           
  Misses        355      355           

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