There is only one file type that uses the .XSLIC extension:
XenServer License File
Software license file used by XenClient, an administration application for XenServer virtualization software; contains a key that registers the XenServer software; includes the expiry date, product code, number of licensed sockets, and other product information.
XSLIC files are typically downloaded through a Citrix Web interface after receiving a "Citrix Licence Delivery" email when the product was purchased. You can install your XenServer license key in XenCenter by highlighting a XenServer instance, choosing, and then choosing your XSLIC file.
Updated: July 27, 2011