I just downloaded recently available files from the C3S site for Sea level daily gridded data for the global ocean from 1993 to present for January 2018.
The files have extensions "vDT2018". For example, dt_global_twosat_phy_l4_20180130_vDT2018.nc
Files I've previously downloaded do not contain the "vDT2018". What does the new extension mean?
Can I combine files previously downloaded without the extension with files with the extension?
The version tag of the sea level products (DT2018) means "Delayed-Time product released in 2018" (previous release was DT2014). The file nomenclature of the product has been recently modified to include this version tag. The physical content of the sea level products available in the Climate Data Store has not changed since June 2018. So you can combine these files with the files previously downloaded without the extension.
This change of file nomenclature is not yet described in the Product user guide and specification document: an updated version of this document is expected by the end of March 2019.