Comment (by Roland Haas):

There is an explanation present since at least git hash 3c44bc84 "Fixed numerous spelling errors, some grammatical boo-boos, formatting flaws (inlcuding gratuitous hyphenation and un-dotted 'e.g.' and 'i.e')" of cactus which is from Mon Jan 5 16:28:24 2004. This is in doc/UsersGuide/ThornWriters.tex and currently reads:

All parameters defined in a thorn’s param.ccl and all global parameters, appear as local variables of the corresponding CCTK data type in Fortran source code, i.e. Booleans and Integers appear as CCTK_INT types (with nonzero/zero values for boolean yes/no), Reals as CCTK_REAL, and Keywords and String parameters as CCTK_STRING (see also note below).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/1072/user-guide-does-not-explain-how-to-use