A SAS compute server (compute tier) is the place where the hard work is done - eg. the workspace server which actually executes your SAS code.
The terminology is slightly developing between SAS versions. Best is you go with the latest "version" as found here:
http://support.sas.com/documentation/cdl/en/biov/64876/HTML/default/viewer.htm#p11gsfehmnv62dn1k5fm3eq2wzd9.htm
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