A massclaim (or mass roleclaim) is an event where every player in the game claims their personal information, including but not limited to their role name, their flavor name, any results they may have, and any players they may have targeted. The actual process may be tailored a little more than that based on what exactly is being claimed, but that's the general idea.
The advantage to massclaiming is that it forces players to drop all pretenses and place final claims down. This reveals the Town's power roles and all helpful results they may have while forcing the scum to stick to a claim they may regret. The disadvantage to massclaiming is that it reveals to the scum exactly who the power roles are.
In addition, massclaim can sometimes be used to break poorly-made setups. This is particularly the case with ill-designed themed games and open games. In open games, finding a breaking strategy can be fairly straightforward. In some themed games, the identities of the scum can become apparent either through analysis of the flavor (especially if the scum have no or weak safeclaims) or through a broken role combination.
As a general rule, Town players prefer for scum to claim early. A scum player near the end of the claim order can falseclaim more effectively with the knowledge of previous roles. For instance, if no Cop has claimed by the end of the claim sequence, a Mafioso can claim Cop with little fear of being counterclaimed. If the Mafioso is near the beginning of the list, that same claim is risky as it is not known whether they will be counterclaimed and therefore outed as scum.
One drawback to the first two methods is that players seem to suddenly disappear for long periods of time once a massclaim begins, and unfortunately, not all of these stalling players are Town. Thus, massclaims have been known to take a week or longer.
In this claim method, the players choose by consensus one of the scummier players to claim first. As each player claims, they declare the next person to claim. The effectiveness of this approach is entirely dependent on how "close" the players are to finding the scum.
As its name implies, the claim order is determined randomly. This is done either with each player posting a dice tag as they claim, or by posting a randomized sequence and claiming in that order. The effectiveness of this approach is, of course, random.
This is rarely used nowadays, at least partly because many moderators' rulesets ban provable randomness such as dice tags.
In this claim method, players are simply ordered to claim in their next post if they have anything of interest to claim - or at least say that they have something. Failure to do so and then claiming will be treated as a scum claim. This forces the scum to gauge how long they can lurk before they can claim, especially as conversation may continue during the claim process. In addition, this method is by its nature fairly quick.
Confirmed Player's Choice
In the situation where a player is known to be town (e.g. an Innocent Child, or a player who was found to be innocent by a Cop who's since died), it's typically most effective to allow that player to determine the claim order. This makes it impossible for the scum to manipulate the process.