Partial DST is SST. DST is Dedicated Service Tools which can only be activated when the system is in a dedicated mode or specially triggered from the control panel while SST is Start Servce Tools which gives you a partial list of DST functions on a live system that you can work with such as disk manager, LPAR configuration on a pre POWER 5 class machine, service tools, etc. These function are mostly for use by your IBM CE to help diagnose and help correct issues.
You can get into DST with the system active. Use option 21 from the control panel as described here.
I’ve only had to do this on systems where operations doesn’t have TCPIP and QINTER autostarting and the console came up varied off, or to verify for an auditor that the DST default pwds have been changed. DST and SST pwds are not necessarily the same unless you’ve made it that way after changing the defaults, and you should always be sure you know the DST pwds.