What are they talking about

So today we learn that the official assessment of the terrorist threat to the UK has been downgraded, from "severe" to "substantial". According to MI5, severe means that "an attack is highly likely" while substantial means that "an attack is a strong possibility".

But what does this mean? What timeframe are they talking about? Without a timeframe it is meaningless. I would say that it remains "highly likely" that there will be a terrorist attack at some point in the future, even if the perpetrators have not even thought about it yet. Surely the threat level must reflect a combination of probability and immediacy.

Bizarrely, the highest level, "critical", means that "an attack is expected imminently". Has the assessment of probability ("expected") changed from "highly likely", or is it the immediacy that changes.

I heard one reporter on BBC describe the previous level (severe) as meaning that it was highly likely that an attack would take place in the near future. This isn't what the system says. But it does perhaps explain why the level has been downgraded. You can't go on forever saying that it is likely that an attack will happen soon.