That slot is used for WWAN cards.  There is some talk that it can be used for an SSD, but only mSATA SSDs are available in this form factor -- which are now obsolete and will be more expensive that you'd think as a result.  Also bear in mind that while a card installed in that slot is useable for storage, it cannot be used as a boot drive.  
I would like to add a SSD to my Latitude without removing my SSHD . is it possible to add it in the Consult manual slot?
Latitude E5540 PCIe SSD upgrade