Isolated Units

[Gregor]

Isolation occurs when you start your turn unable to trace a path back to any supply source.

A path is blocked by enemy units, ZOC (if not a friendly unit in the ZOC), impassable terrain and mines.

Think that's the basic of it.

The test are only at the start of your turn and units keep their status until your next start of turn when a new test is run.

Edit: from the user manual:
Supply Determination
When supply issues are being resolved for a unit in a given hex, the supply
value that applies to that hex is used. If there are no Supply Sources in the
scenario for that side, then the Global Supply Value is used for this purpose.
Otherwise, the Supply Source of highest value affecting that hex is used. If the
unit is Isolated, then the supply value in the hex is 0. Otherwise, the unit must
trace a line of communication (a line of hexes free of enemy units and their
Zone-of-Control crossing rivers only using bridges or ferries) to any Supply
Source hex or map edge ground hex when Supply Sources do not exist. In the
absence of Supply Sources, any map edge ground hex is essentially a Supply
Source with that side’s Global Supply Value. If there are Supply Sources used
for that side in the scenario, then the unit must trace a line of communication to
a Supply Source rather than ground edge hexes. When determining this line of
communication, the presence of friendly units negates enemy Zones of
Control.