In output from `apt update` what do 'InRelease' and 'Release' refer to?

Question

Here is an example showing the 'InRelease' and 'Release' line suffixes:

# apt update
Hit:1 http://security.debian.org stretch/updates InRelease
Ign:2 http://dl.google.com/linux/chrome/deb stable InRelease                                                                                                                                                                                 
Hit:3 http://sfo1.mirrors.digitalocean.com/mariadb/repo/10.2/debian stretch InRelease                                                                                                                                                        
Hit:4 http://mirrors.ocf.berkeley.edu/debian stretch-updates InRelease                                                                                                                                                                       
Ign:5 http://dl.google.com/linux/earth/deb stable InRelease                                                                                                                        
Ign:6 http://ftp.uk.debian.org/debian stretch InRelease                                                                         
Hit:7 http://dl.google.com/linux/chrome/deb stable Release               
Hit:8 http://ftp.debian.org/debian stretch-backports InRelease           
Hit:9 http://dl.google.com/linux/earth/deb stable Release                
...

I know what a Release is. Although a web search for InRelease didn't turn up much.

Answer 1

InRelease files are equivalent to Release files with the exception that they contain an inline GPG signature, whereas validating Release files requires downloading a separate Release.gpg file. Having the signature in line avoids race conditions when downloading.

This FTP master meeting summary contains this feature’s announcement, with a brief description.

Since the title asks about them too, I’ll mention that Release files contain distribution metadata and the checksums of the index files.

Answer 2

Another reference to this is here:

InRelease files are signed in-line while Release files should have an accompanying Release.gpg file.

https://wiki.debian.org/DebianRepository/Format#Origin