Explanation
https://help.fortinet.com/fmgr/50hlp/56/5-6-2/FortiManager_Admin_Guide/1000_Device%20Manager/1200_ins There are 4 main wizards:Add Device: is used to add devices to central management and import their configurations.
Install: is used to install configuration changes from Device Manager or Policies & Objects to the managed devices. It allows you to preview the changes and, if the administrator doesn't agree with the changes, cancel and modify them.
Import policy: is used to import interface mapping, policy database, and objects associated with the managed devices into a policy package under the Policy & Object tab. It runs with the Add Device wizard by default and may be run at any time from the managed device list.
Re-install policy: is used to perform a quick install of the policy package. It doesn't give the ability to preview the changes that will be installed to the managed device.

Explanation
https://kb.fortinet.com/kb/documentLink.do?externalID=FD38828

Explanation
https://help.fortinet.com/fos50hlp/52data/Content/FortiOS/fortigate-WAN-opt-52/web_proxy.htm#Explicit2 The explicit proxy does not limit the number of active sessions for each user. As a result the actual explicit proxy session count is usually much higherthan the number of explicit web proxy users. If an excessive number of explicit web proxy sessions is compromising system performance you can limit the amount of users if the FortiGate unit is operating with multiple VDOMs.

Explanation
Route reflectors help to reduce the number of IBGP sessions inside an AS. A route reflector forwards the routers learned from one peer to the other peers. If you configure route reflectors, you dont' need to create a full mesh IBGP network. All clients in a cluster only talck to route reflector to get sync routing updates. Route reflectors pass the routing updates to other route reflectors and border routers within the AS.