The topic ID is the unique alphanumeric identifier of a topic. It must be unique because it is used to reference the topic for hyperlinks and by programmers accessing the topic from applications.
The help context number is an additional unique topic identifier. Context numbers are optional for making calls to normal topics and required for making calls to field-level popups displayed directly in applications. Most programmers now use normal IDs for calls to topics because they are human-readable, which reduces errors.
You can view and edit topic IDs and help context numbers in the Topic Options tab on the left of the main editor window. For more background information see IDs, Context Numbers and Keywords in the Reference section.
Accented characters, special characters and high-ASCII characters are not permitted in IDs. For maximum compatibility with all output formats only use a..z, A..Z, 0..9 and _ in topic IDs. Topic IDs are never seen by the user.
IDs, Context Numbers and Keywords (Reference)