One flight or two?

This might sound like a simple question, sorry if i've overlooked something.

Does the flight have to be a CONNECTING layover flight or can I purchase two separate flights (within the allocated time period)?

For example, if I book my FIRST flight from Hong Kong, direct to Beijing, remain in Beijing for 72 hours, and then take a SECOND flight Beijing to London, does this still allow the 72 hour transit. Or does it have to be ONE connecting flight from (for example) Hong Kong - Beijing - London (with a period of time between each flight). Is the simple showing of an ongoing flight enough?