Abreviations on pcb

before I put my anushri synthesizer into a long term enclosure , i would like to add any additional i/os , I am aware that there are additional connections on the pcb , however i do not know all of the abbreviations.
i would like to know what EN , CLK ,SER stand for what aspect of the vcf the vcf connection control.

Thanks

None. These are digital lines from driving a 595 shift register - just like the Output Expansion Port of the Shruthi.

Thanks