What exactly policy installation do?

[appyens]

I want to know about policy installation in publish API context.

1. What if I publish the changes but not installed policy.
2. Is it mandatory to install a policy for changes to take effect?
3. In publishing, I can publish any session by providing UID, is this the same with policy installation?
4. Can I install it for a particular session?

Please help!!

[PhoneBoy]

A publish action will commit the changes made during the current session to the management database.
You can potentially publish multiple times before you perform an Install Policy, which is required for the changes to take effect on the Security Gateway.
Publish only applies to the session you are currently on, but you can "take over" an existing unpublished session and publish that.
Install policy takes the current management database with all changes published to that point (by any session) and pushes to the specified gateway.

[HeikoAnkenbrand]

Hi @appyens 

@PhoneBoy has already mentioned it well.
Here you can find more information about policy installing:
R80.x - Policy Installation Flowchart