What does the 6363 stamp on my son's passport mean?

Question

My son wanted to return to the UK from Romania and at the border the officer put a stamp on his passport and sent him back to Romania.

The number on his passport is 6363.

Answer 1

A stamp with a “cross" drawn through it means leave to enter the UK was refused. 6363 is just the number of the officer that processed your son.

There’s an example of such a stamp in this question

Answer 2

If the 6363 is printed inside the stamp that the UK Immigration Officer used, then it is a number used to identify that particular officer.