Most platforms treat time as one global clock. Markets do not work that way.
A euro pair at 03:00 UTC and the same pair at 08:00 UTC are, for every practical purpose, two different instruments: different spread, different depth, different response to the same size.
What changes across a session
- Spread, frequently several times wider outside the instrument's primary session.
- Depth. Size that fills comfortably at the London open can move the price materially in a thin Asian session.
- Volatility character — not just the level, but whether moves trend or revert.
- Correlation, which often only holds during overlapping sessions.
The consequence for limits
A position size that is conservative during the primary session can be aggressive six hours later without anything about the position changing. So a static exposure limit is wrong for most of the day: too loose in thin conditions, or needlessly tight in liquid ones.
Two things follow.
Limits should be set for the thinnest conditions in which a position will actually be held. Not the average, and definitely not the conditions in which it was opened.
Holding across a session boundary is a decision. Usually an unexamined one. The question "am I willing to hold this into Asia" has a different answer from "do I want this position", and most risk frameworks never ask the first one.
How it is modelled
Session windows are configured per instrument rather than globally, because "active session" means something different for USDJPY than for GER40. The intelligence layer reports which sessions are open, and how the current session compares with that instrument's typical behaviour — the useful reading is not "volatility is 14%", it is "volatility is high for this instrument in this session".
Where this shows up operationally
For a broker, session awareness is not a nicety. It determines when to widen, when to reduce available leverage, and when an unusual spread is a market condition rather than a feed problem. A platform that cannot tell those apart will either alert on nothing or alert on everything.
Related: Global Markets · correlation is a window