What does the number after each command that listed by the apropos command mean? (1-8, pm, ssl, etc)

Question

I was trying to complete an article on apropos command. When I run the apropos command, it lists out something like this:

fahim@Fahim:~$ apropos delete
argz_delete (3)      - functions to handle an argz list
crlutil (1)          - List, generate, modify, or delete CRLs within the NSS ...
DELETE (7)           - delete rows of a table
delete_module (2)    - unload a kernel module
flatpak-remote-delete (1) - Delete a remote repository
git-branch (1)       - List, create, or delete branches
git-replace (1)      - Create, list, delete refs to replace objects
git-symbolic-ref (1) - Read, modify and delete symbolic refs
git-tag (1)          - Create, list, delete or verify a tag object signed wit...
groupdel (8)         - delete a group

After each of the listed commands, there's a number in parentheses. What does that number mean? What's the use of it?

Edit: I've seen the answer on man page. To check out that all the numbers are between 1 to 8, I tried this command and some of the results were:

fahim@Fahim:~$ apropos a
YAML::LibYAML (3pm)  - Perl YAML Serialization using XS and libyaml
x509 (1ssl)          - Certificate display and signing utility
x509v3_config (5ssl) - X509 V3 certificate extension configuration format
X25519 (7ssl)        - EVP_PKEY X25519 and X448 support

The answer about the numbers make sense, but what about these 3pm, 1ssl, 5ssl, 7ssl?

Answer 1

The table below shows the section numbers of the manual followed by the types of pages they contain.

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions, e.g. /etc/passwd
   6   Games
   7   Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]

I cut the above text from executing the "man man" command.