Notepad++ uses these symbols to represent control characters or non-printing characters.
Control character - Wikipedia
A control character or non-printing character is a code point (a
number) in a character set, that does not represent a written symbol.
C0 and C1 control codes - Wikipedia
STX - Start of Text - First character of message text, and may be used
to terminate the message heading.
SOH - Start of Header - First character of a message header.
GS - Group Separator - Can be used as delimiters to mark fields of
data structures. If used for hierarchical levels, US is the lowest
level (dividing plain-text data items), while RS, GS, and FS are of
increasing level to divide groups made up of items of the level