This question should be directed to the campaign managers and not the admin. The forum does not run signature campaigns and as far as i know, they also do not moderate it.
Campaign managers will try to adopt what they think is a good criteria to select participants in their campaigns and it can include a lot of things and it also depends on the type of campaign, is it a casino, mixer, instant exchange, etc. However i don't know if it is important for a manager to make such criteria public, it can affect the posting patterns of members who may try to post to meet those criteria. Lastly, just communicate freely on the forum, if you don't spam, you have a chance to get in a campaign.