The documentation for multicol mentions on the first page:
Note: This package is released under terms which affect its use in commercial applications. Please see the details at the top of the source file.
However, the archive says it's distributed under the normal TeX license, which doesn't seem to put any conditions on its use (only on redistribution and modification).
Which is correct? What should I keep in mind if I want to use multicol in e.g. a book?
As the line you quote says, the details are in the source file. It is distributed in LPPL with an additional clause:
% \iffalse meta-comment % % % This file is part of the Standard LaTeX `Tools Bundle'. % ------------------------------------------------------- % % It may be distributed and/or modified under the % conditions of the LaTeX Project Public License, either version 1.3c % of this license or (at your option) any later version. % The latest version of this license is in % https://www.latex-project.org/lppl.txt % and version 1.3c or later is part of all distributions of LaTeX % version 2005/12/01 or later. % % The list of all files belonging to the LaTeX `Tools Bundle' is % given in the file `manifest.txt'. % % \fi % % \iffalse This is a METACOMMENT % %% Package `multicol' to use with LaTeX2e %% Copyright 1989-2018 Frank Mittelbach %% %% In addition to the terms of LPPL any distributed version %% (unchanged or modified) of multicol has to keep the statement %% about the moral obligation for using multicol. In case of major %% changes where this would not be appropriate the author of the %% changed version should contact the copyright holder. %% %% %% Moral obligation for using multicol: %% ------------------------------------ %% %% Users of multicol who wish to include or use multicol or a modified %% version in a proprietary and commercially market product are asked %% under certain conditions (see below) for the payment of a license %% fee. The size of this fee is to be determined, in each instance, %% by the commercial user, depending on his/her judgment of the value of %% multicol for his/her product. %% %% %% The conditions for this are as follows: %% %% The producer of a proprietary and commercially market product %% that involves typesetting using multicol is asked to determine %% the value of a license fee for using multicol if %% %% - the product is a document and the producer has decided to %% include multicol to typeset (parts of) the document or has %% directed the author of the document to include multicol (for %% example, by providing a class file to be used by the author) %% %% - the product is a LaTeX class or package that includes multicol %% %% %% There is no moral obligation in case %% %% - the product is a document but producer has not directed %% the author to include multicol (in that case the moral obligation %% lies with the author of the document) %% %% - the product does not involve typesetting, e.g., consists, for %% example, of distributing multicol and its documentation. %% %% - the product is not proprietary, i.e., is made available as free %% software itself (which doesn't prohibit its commercial marketing) %% %% - multicol is used for non-commercial purposes %% %% %% Determining a license fee might result in a license fee of zero %% (i.e., no payment) in case a producer has determined that the use %% of multicol has no enhancing effect on the product. This is a %% plausible scenario, i.e., in the above two cases the producer is %% only asked to evaluate the value of multicol for the product %% not for the payment of a license fee per se (which might or might %% not follow from this evaluation). %% %% The license fee, if any, can be payed either to the LaTeX3 fund %% (see ltx3info.txt in the base LaTeX distribution) or to the author of %% the program who can be contacted at %% %% [email protected] %% 