The Windows 8 version of Tasks supports Exchange and Telerik Cloud sync and Windows Phone 8 version of Tasks supports the listed above Exchange and Telerik Cloud + Google sync.
As you have already found, there is no limitation of the number of the devices from one OS- only one license is required.
Regarding the provider field into View/Edit task/category/project
- please, notice that for a task, related with a defined provider, only the associated with that provider projects and categories will be listed into the corresponding fields of the task
- also, it is important to mention, that once a provider has been set to a task/project/category, it could not be changed
- when a provider is removed, there are two options- to be deleted the provider data from the device or to be kept and associated as local data
- we absolutely agree that a help page will be useful and we are already working on it
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