Tryton application platform - sale subscription asset module

 Tryton is a high-level general purpose application platform. It is the base
 of a complete business solution as well as a comprehensive health and hospital
 information system (GNUHealth).
 .
 The sale subscription asset module adds the notion of asset to the sale
 subscription module.
 .
 Two fields are added to the *Service*:
 .
     - *Lots*: All the lots that can be used when providing this service
     - *Available Lots*: A subset of the preceding field displaying the
       available lots
 .
 On the *Subscription Line*, it is possible to specify the lot to use. When the
 subscription will be running this field will become required for service with
 lots.
 .
 On the *Lot*, the field *Subscribed* will point to the line of the subscription
 currently using this lot.