In practice
Know where your platform ends a session — inactivity timeout, midnight, a change of campaign source — because it changes every rate derived from it.
Compare sessions to users before reading engagement: the same total can mean many visits from few people or the reverse.
Why it matters
It is the denominator for conversion rate, pages per session and most engagement reporting.
A definition change between platforms breaks year-on-year comparisons in a way that looks like a performance change.