Remote Management

Persistence of systems is provided by overrides and updates managed remotely.

What Changes Why Necessary
Level 1 Reissue New medium (CD or stick) New Debian release or package cache
(packages can also be updated in 2, 3, or even 4)
Change to 'hook' (very unlikely)
Level 2 Reboot New instance from online /custom Power or hardware failure or system crash
All-system startup changes
Registry changes
Level 3 Update Online /custom to existing instance System-specific fixes or development,
of database, applications, web publishing, etc
Level 4 Patch Just existing instance One-off - persistence is not needed or wanted

Factors determining whether media are reissued include stability, required functionality, and security of OS software.
2017-09 update: new media can now be generated on live instances; the management of package caches is fully automated.

The online version of /custom overrides the instance in 2, or updates it in 3. Essentially the same thing is happening but there are subtely different implications. During a restart (reboot) initialisation scripts are deployed, whereas reinitialisation may be required after an update. An automatic update occurs overnight or one may be requested at any time.

Before this release 'patching' was the only way of making many important changes. Rebooting was either impracticable or undesirable, but often necessary. And the only system persistence was provided by the boot medium.