A newly created thread may be designated as "Under Construction" by the person starting it. Occasionally, presentation of a subject may be complex and lengthy, requiring a period of time to prepare over an initially unknown number of posts. Unlike the case of a document prepared at home or in the office, then submitted when complete, a forum presentation may be assembled online and be visible to the general public while it is in work.
General Membership responsibility
As a courtesy, members are expected to observe and respect a declaration of a thread being under construction for however long the status is in effect. In a thread geared towards presentation, it is disruptive to interrupt the flow of presentation with other posts. Once a thread is opened for general discussion by the author, other members are then free to post.
Thread Author responsibility
Thread authors are encouraged to have their act together somewhat before starting a thread to minimize the time in which it's off-limits to others, though there is no strict limit. Authors have the responsibility to prominently declare the thread as UNDER CONSTRUCTION so it is easily discernable at a glance. Any declaration in the first, original post is sufficient to put the status into effect, but it is preferred the author use tags (described below) to help ensure visibility. If a thread should continue to be in work after many posts (or multiple pages), additional markers should be placed periodically.
The thread author has responsibility for making their intent clear at the outset.
Thread authors can, at their discretion, allow specific cases of "unwanted" interruption to remain if it turns out they like where the discussion went. Because of this flexibility, and because a moderator may not see every violation, thread authors (and only they) must report any violations by using the 'report post' function.
Adherence to this policy is by civil agreement, and enforcement is by manual moderator intervention. There is no means to provide a special thread status to the general membership which only allows one person to post. Members who do post out of turn in an under construction thread will NOT receive warnings (unless it's flagrant or abusive) but will likely find their post(s) moved to somewhere else where they will make little sense standing alone.
Mistakes will happen, that's why the violation is not an infraction. Try to be mindful, though, as it makes for better threads.
Flexible Posting Status
Just because a thread is under construction does not mean the author will reject any or all posts submitted by other members. The original poster may designate other members as collaborators or excuse violations on a case by case basis as noted. No one should take offense if some interruptions are tolerated where others aren't.
A thread author may declare a thread (or a thread detailing an ongoing project) as under construction, but still allow or even encourage input from others. The designation is simply to inform viewers that additional content pertaining to the OP is forthcoming, and the "OP" itself should be considered incomplete until the sign is removed. Discussion will be entertained, but with the understanding the presentation is ongoing. Alternately, a number of posts may be 'reserved' for future use by the author, and replies allowed immediately even while the block of posts is in work.