Those variables are defined in the S_WindowCovering1.xml
file
<?xml version="1.0"?>
<scpd xmlns="urn:schemas-upnp-org:service-1-0">
<specVersion>
<major>1</major>
<minor>0</minor>
</specVersion>
<actionList>
<action>
<name>Up</name>
</action>
<action>
<name>Down</name>
</action>
<action>
<name>Stop</name>
</action>
</actionList>
</scpd>