When Adding Products to Shopping campaigns in S&O for either Google or Bing, you have the option to select the following build types:
Auto Build: S&O automatically sets which Ad Groups products are to be added to. At maximum, an Ad Group created by S&O will contain 14,000 products before 1 or more Ad Groups would have to be created for any remaining products
Manual Build: Allows you to control exactly which Ad Groups to have products added to. In other words, selecting this unlocks the Ad Group selection drop down so you can select them yourself. This is used primarily when you have multiple Ad Groups in a Campaign for specific grouping strategies (ie by Brand).