Another rule are boards where if a participants makes a posts it will not the counted. I am going to ask these questions out of curiosity. First question, why are there boards where the signatures cannot be displayed or aren't visible on the forum?
By your rank, I assume you know what signature means in the forum. The Forum per se is a place for sharing ideas on bitcoin but not limited to only bitcoin, it captures alt coins as well as political and economical discussion. Signature is an advert mechanism for companies or related entities where users are required to make certain number of posts in order to get paid and by implications, it leads to numerous post with less content and meaningful conversation. Consequently, signature are disable in the board where serious discussions are being discuss in order to keep the board clean and convo relevant.
Second question is why do campaign managers exclude certain boards for posting and others where posting is allowed?
Some boards are excluded thou this depends on each campaign manager preference. Some board have less post simply because members are not interested in them and the purpose of signature campaign is to create awareness about a particular site or whatsoever with the aim of getting traffic by spreading it where majority of Forum users are active in.