Here's how I handle posts that can be in multiple categories.
1) Create the categories editor
I use the excellent Grid Class Key, but you could use articles and even just resources in a container
2) Create a categories tv and assign it to your post template
Input Type = Listbox (mulit-select)
Input option = @SELECT pagetitle, pagetitle FROM modx_site_content WHERE parent=20
output type = delimiter
delimiter = ||
*change 20 to the id of your categories container
3) List the relevant posts in the categories template
[[getResources? &parents=`21` &tpl=`postTpl` &limit=`99` &processTVs=`1` &includeTV=`1` &includeTVList=`categories` &tvFilters=`categories==%[[*pagetitle]]%` &depth=`0`]]
* &parents is the id of your posts container
4) Create a categories menu
[[Wayfinder? &startId=`20` &level=`1`]]
[ed. note: eighthday last edited this post 10 years, 2 months ago.]