# Level II market data (equities)

I have been playing around with some equities historical market data and I understand that different exchanges have different definitions of what Level II (Limit Order Book) data means. What I'm interested in knowing is when/how often is the data updated.

In fact, I see that some exchanges (like NYSE with their OpenBook data) aggregates and refreshes the data every second, while others don't specify at all when/how but appear to do so at irregular time intervals. The question is: is the data updated at every occurrence of an event (such as the submission of a Limit Order, execution of a Limit Order or cancellation of a Limit Order, etc...) or is there some other way?