With ConfigMgr 2012 there are 2 packages used to install the ConfigMgr client. The first package which is called the Configuration Manager Client Package can be found under the Packages node of the Software Library and is used during OSD in the Setup Windows And ConfigMgr step to install the ConfigMgr client. The other package, which is hidden is called Configuration Manager Client Upgrade Package and this package is used when the Client Upgrade feature of ConfigMgr 2012 is enabled. The fact that this package exist can only be determined by looking at the Content Status under Distribution Status in the Monitoring pane for example.

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The Client Upgrade functionality provided in ConfigMgr 2012 can be used to upgrade existing ConfigMgr clients already assigned to your Site to a new version. The functionality is not suitable if you have many clients, because it’s an on/off option without any further control in terms of scheduling or collection scoping.

If for some kind of reason the Configuration Manager Client Upgrade Package gets deleted, you cannot recreate it by creating a new one from a package definition file, like you could do with the Configuration Manager Client Package.

Normally when the Configuration Manager Client Upgrade package is deleted, this should be detected and  should be regenerated automatically, but if it doesn’t you can create a empty file with the name client.acu and place that file in the inboxes\hman.box folder at the top-level site (either your CAS or PSS). After that check your hman.log to check whether the Client Upgrade package gets updated.

Reference: http://social.technet.microsoft.com/Forums/en-US/configmanagerosd/thread/cf091a72-7a86-4e85-a2d3-b55094128364/