The Ruler of the East is a 
Russian decision regarding 
China and in particular the substate 
Manchuria. The requirement is, that Russia has at least 10 prestige and owns Outer Manchuria, which it usually does if it has taken the decision The Peking Convention.
It gives 5 prestige and renames the state Outer Manchuria to Primorye. The 5 provinces in Outer Manchuria / Primorye are also renamed to Russian names.
If China reclaims the province, they will have no way of renaming it to Outer Manchuria again, and even under Chinese rule the province will forever be Primorye
The in-game text says: In july of 1880 the port of Vladivostok or 'Ruler of the East' was founded near the Korean frontier. It would become a major Russian trade port and shipbuilding yard. Eventually, it would become the focal point of an intercontinental railroad as well as a major scientific, cultural and industrial center in its own right. The Czar dreamed that the city would become a hub connecting the west and the east. Perhaps the railroad to Vladivostok will eventually replace the Suez Canal as the bearer of goods and raw materials between the east and the west?
The event text will mention the Suez canal, even though it might not have been build yet.