I am sorry if this topic has been beaten to death, but I am trying to get my head around this, so please bear with me

The blockade boxes I understand. They are straight forward. I also understand that blockade squadrons are the best to use for blockading coastal cities. So good so far. I also understand that you need to blockade all entrance points to the harbour of the city you wish to blockade. Again, fully understood.
Now for where I am completely confused:
Does one blockade squadron represent one unit for blockading purposes? Example: 4 blockade squadrons in one fleet = 4 units?
I assumed as much, however, taking Wilmington, for example. It's harbour has two entrance points, each requiring 4 'units' to effectively blockade the port. I have placed one fleet made up of 4 blockade sqaurdron's in each of the designated entrance points.
Now when I hover over the two sea areas, that prior to having the fleets in them stated that '4 units are required to blockade this entrance' it now states that I now require 2 'combat units' to blockade.
To recap: What constitutes a 'unit' for blockade purposes? What make up of squadron/fleets?
