Content type with a field of multiple values of other content type (1-n relation) What should be the approach for the common cases where we need a content type with a field of multiple values of other content type? The typical "1 invoice as n invoice lines" where invoice is a content_type and *invoice_line* other content type.
I did some research and I'm confused because there are: Relation, References, Entity References... 
(For Drupal 7)

Thank you.
The Entity reference would be the best choice for your requirement. It is also a successor of the References module.


  References will most probably be deprecated in the near future in
  favor of Entity Reference, which should probably be considered first
  on fresh D7 projects.


The Relation module is more suitable for complex relationship. You can see this clearly in its module page:


  Relation is an API module and storage model for both simple and the
  most complex relations between entities. The module can handle both
  directional and symmetrical relations very well.
  
  Relations are entities and can therefore have fields. This makes
  Relation the most flexible and powerful relation model out there.