Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Mobile/7B405

The components of this string are as follows:

Mozilla/5.0: Previously used to indicate compatibility with the Mozilla rendering engine.
(Windows NT 10.0; Win64; x64): Details of the system in which the browser is running.
AppleWebKit/537.36 : The platform the browser uses.
(KHTML, like Gecko): Browser platform details.
Mobile/7B405: This is used by the browser to indicate specific enhancements that are available directly in the browser or through third parties. An example of this is Microsoft Live Meeting which registers an extension so that the Live Meeting service knows if the software is already installed, which means it can provide a streamlined experience to joining meetings.