WARNING: This page is about a different car, the 2018 BMW X4, 2017 BMW X4, 2016 BMW X4, and 2015 BMW X4. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Frame (000226)
Courtesy of BMW OF NORTH AMERICA, INC.
NOTE:
(Side frame) Only required once for constructing the base frame Deletion, only available via toolset