How to correctly name the separator string, which is used to separate different entities in text formats such as "key-value"?

Question

Parsing

More precisely, in my case, this is not exactly a line, these are 2 lines, one at the beginning of the entity, the other at the end.
How to call them in this case?
Is it possible to call it boundary by analogy with the multipart body of an HTTP request?
The format looks like this:
---BEGIN: Entity1---
Key11: Value11 Key12:
Value22
---END: Entity1---
---BEGIN: Entity2---
Key21: Value21 Key22:
Value22
---END: Essence2---

Answer 1

I think markerwould be the right word.
This word is not uncommon in the context of data format.
Relevant results with boundary are harder to find .
A section marker would probably be a section marker
. For example, you could name the identifiers section_begin_markerand section_end_marker.