Oftentimes it is useful to explicitly mark the end of a definition, remark, or any other logical unit in the text. This can be done by typographical tricks like font changes (italics are common for theorems) or additional vertical whitespace. However, this in my experience lacks the clarity provided by an explicit end marker.
The most frequent use of the Tombstone appears to be for marking the end of a proof. In this book however, the tombstone is used as an end marker for any numbered (and thus referenceable) unit. I find this an elegant solution. Some people have objected to this since according to them, the tombstone should only be used to mark the end of a proof.
According to Paul Halmos (in "I Want to Be a Mathematician") on the other hand, the tombstone "is used to indicate an end, usually the end of a proof." So it is only usually the end of a proof, not always.
Dijkstra uses "(end or remark)" or "(end of proof)".
Could someone please point me to other works that use explicit end markers (or phrases) for certain units in the text?