Think of it this way: normally a database entry is represented as a row. You could also represent this exact same data as a list of triples: primary key, attribute name, and attribute value. Same data, different representation.

Datomic stores its data in a series of quadruples: primary key, attribute name, attribute value, and transaction. These facts are append only; when data is normally deleted the transaction includes the fact that the data is being deleted, not added. Under the hood Datomic processes these facts to produce the current state of the world, but the old data is there if you ask for it.

Excision deletes facts from the database, which both violates a lot of assumptions about how the database works, and permanently removes the data. In the process of excision it leaves a single record to indicate that something was removed, without clarifying what. It’s something they only recommend for regulatory compliance, as it eliminates a lot of the value in the database.