Depicting molecules by Natta projection
Cram bonds are synonymous to the "wedged" bonds or, in this case, to "hollow wedged" bonds, which IUPAC [1, p. 1910] describes as follows (emphasis mine):


  There are a variety of obscure conventions for depicting arbitrary mixtures of diastereoisomers. The most common (but still obscure) of these conventions says that bold and hashed wedges should be used for absolute configuration, unwedged bold and unwedged hashed lines represent relative configuration and racemic character, while hollow wedges and dashed lines represent relative configuration only.


A bit more context is given in [2] (bond type 4):


  Central to the correct perception of a stereounit’s topography is the stylized generation of a vanishing point, realized by the graphic stereodescriptor, or
  stereobond, commonly called a “wedge” as illustrated in Figure 1 as 1a and 1b. Three such wedge symbols as in 3, 4, and 6 are in use, each of them capable of portraying the ligand Z in 1 and 2 as being located in the front (3 and 4) or behind (6) the picture plane. In an apparent search for further simplification, line symbols 5, 7, and 8 were invented as wedge substitutes.
  Lacking differentiation of their termini, line-type symbols appear, a priori, to be inherently incapable of serving such a purpose; nevertheless, they provide a useful function in the context of a convention. Accordingly, the stereogenic unit to which the line-type bond symbol is attached is considered to lie in the picture plane so that the symbol printed in bold (5) reveals its ligand Z in front of that plane, whereas the hashed symbols (7 and 8) are intended to show that the ligand is behind it. In other words, symbol 5 portrays the stereocenter at the vanishing point and hence functions such as 3 and 4.
  
  
  
  Figure 1. Perspective drawings of stereocenters with commonly used stereobonds.


References


Brecher, J. Graphical Representation of Stereochemical Configuration (IUPAC Recommendations 2006). Pure and Applied Chemistry 2009, 78 (10), 1897–1970. https://doi.org/10.1351/pac200678101897.  
Maehr, H. Graphic Representation of Configuration in Two-Dimensional Space. Current Conventions, Clarifications, and Proposed Extensions. J. Chem. Inf. Comput. Sci. 2002, 42 (4), 894–902. https://doi.org/10.1021/ci025518w.