I have been trying to figure out how to get anti matter containment research and went searching through the X Com Files Wiki. As far as I could tell you get it from interrogating an alien engineer/leader. It's categorized under 'getOneFreeProtected' and this is where I got confused.
I didn't understand what it meant so I checked the Ruleset and this is what I found.
A list of additional "bonus" research projects that may be granted when completing this project. Each topic in the list is "protected" by another topic, which allows you finer control over when the player gets the bonus.
For example: you could divide the topics into early-game, mid-game and late-game and protect each group by some "gateway topic".
getOneFreeProtected only works when the research project is completed in a Laboratory. All other means of obtaining a research topic ignore getOneFreeProtected.
I still don't understand how it works. Any help would be appreciated.