When talking in general and it's not stated, I will expect and think that most measure from the BB center to the top of the saddle about where your sit bones go. I'm not going to worry about precision, because once I've got the saddle set by that measurement I'll tweak it up and down over several rides till it is where I want it by what I feel and not by what some formula or other thing says it should be.
My preference though is to measure from the top of the saddle where the sit bones go to the top of the pedal when it is furthest away. That lets me know a measurement I can quickly set any saddle on any road bike to and takes into account the length of the crank arm. This works well for me when I'm shopping for a bike as I like a 165mm crank, but most bikes my sizes have a 172.5 - 175 mm crank on them. So it gets my saddle more correct for that longer crank arm when trying out a bike.
Many other times I hear someone say anything about saddle height along with a measurement, I just confirm with them where they are measuring from. Doesn't hurt to ask just to be certain.