What does STX, SOH, and GS mean in Notepad++ output?

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 beneath it.

SOH is Start Of Header;

STX is Start of TeXt;

GS is Group Separator.