There are cases where people have some sort of acknowledged relationship with a manufacturer and are sharing inside information about niche products, and there are situations where a member has gotten some very modest benefit like a free sample of a couple of rolls of film and are reporting their tests, and there are situations where people are trying to establish a new small business and are promoting their new service or are doing some market research but don't have the capital to advertise yet, and there are collectors of equipment who trade heavily in the classifieds such that it seems like they may be dealers though we know that dealing in old cameras isn't their primary source of income, and these are all okay, as long as everything is clear and above board.
What we're concerned about is underground marketing by established companies as a strategy that can have a negative effect on the APUG community ethos. This might include, for example, members shilling for a manufacturer, without acknowledging that they have a formal or informal relationship with that manufacturer. A formal relationship might be something like serving on an advisory board. An informal relationship might be something like getting a brick of film for being a loyal supporter, and not revealing that you've received such benefits. "Shilling" might be starting a thread about a new product made by the manufacturer that the user has a relationship with, or bashing a competing product.
Legitimate advertisers on APUG are asked not to discuss competing products, because public fights between manufacturers on the forum tend not to help anyone and just bring down everyone's enthusiasm for their products. Legitimate advertisers are also asked to keep commercial discussions to their dedicated sponsor forums, except for responding to direct queries about their products in the regular forums. If a user with an informal relationship to a company starts or joins a thread in the regular forums for the purpose of enabling an advertiser to promote their product outside their dedicated sponsor forum, that would constitute "shilling."