Usually, "Schachtelbeteiligungen" are related to a specific percentage holding that is a threshold. The question is what the threshold is for: there used to be a taxation threshold where a participation was considered "material" from 10% upwards. This has been changed to 1%, so I doubt whether the term is related to this. Alternatively, a "Schachtel" could be related to certain shareholder rights obtained with a certain participation level (e.g. a blocking minority).

As I said in my peer grading comment, it's almost impossible to find an exact translation without more context.