I am using your uc_hosting module, and I want to understand, does this module creates site install task after product is purchased?

There may be some adjustment to UI needed to make this clearer, but the short version is "Yes".

Basically, on a product with the site feature activated on it, you get a little check mark saying "Create my site later" - if that is left unchecked, users should be sent to the aegir site creation form before going on to checkout. After payment completion, the information inputted in the site creation form is used to create the new site.

Regardless of whether or not they enter their site info during ordering, their client's quota is incremented.

In the case of a product kit with multiple sites included, only the first site can be created in this manner. (For the time being)