What is "Collector-Emitter Sustaining Voltage”

This document entitled "Electrical Ratings and Characteristics of Power Semiconductor Switching Devices" contains probably the best picture of this fairly obscure-to-most-engineers phenomena: -

Basically it's to do the the collector emitter breakdown voltage with the base open circuit (it's most vulnerable). The graph above shows 5 scenarios of the base bias ranging from open circuit to heavily reverse biased. Any BJT will behave like this. Note the red box along the voltage axis is the value of \$V_{CEO(SUS)}\$. It's also known as V(BR)ceo in other data sheets or BVceo in the graph below: -

Is the breakdown voltage, take exemple the MD2001Fx datasheet, if you see Figure 2 what they calling the collector-emitter sustaining voltage (700V) is the collector-emitter breakdown voltage.
This is a definition for what they call Collector-Emitter sustaining voltage.