Explained SCPs are commonly articles about anomalies that are completely and fully understood to the point where their effects are now explainable by mainstream science or phenomena that have been debunked or falsely mistaken as an anomaly.
SCP articles that have not yet been assigned an object class may be labelled as Pending. This is used to indicate that the Foundation is actively researching the anomaly, but doesn't have enough information yet to give it an object class. This is a deliberate decision to emphasise that a story is ongoing, and isn't the same as the author not knowing which object class to assign!
Esoteric Object Classes, also occasionally referred to as Narrative classes, are Object classes that do not fall into any of the above sections. They are generally only used once and are created to further the narrative in a particular SCP. It is highly recommended that SCPs use one of the standard Object Classes listed here. While some authors choose to introduce exceptions to these rules, they are only very rarely done and need to justify their existence and placement. Many site members will downvote for non-standard Object Classes if used without merit.