sst_skin Used with sst_update

Hello.
I know that sst_skin calculates SST.
I know that sst_update gives SST.
But,
I found an example where sst_skin and sst_update are used together in the user's guide. In this case, is SST the input data? Is it output data? Other than that?

Hi,
sst_skin calculates the very top layer of the ocean that fluctuates with the diurnal cycle. sst_update updates the SST temperatures, which is about 10 meters in depth, and can take many days to fluctuate. Most people who do long simulations want to use both, as they can both impact the results. SST data should be input through the WPS process, and then following calculations new values will also be output. sst_skin will update and output an SSTSK variable.

I will use them as needed. I will also look at the difference in results. Thank you very much.