What is the meaning of a flashing white signalling light in Aldis Lamp?
The signal you are talking about relates to vehicles (flashing whites can also apply to aircraft, but then it has a different meaning).

Local instructions refers to the local written rules/procedures applicable at the specific airport. In order to be allowed to drive on the maneouvering area, the vehicle driver will have undergone training and have read the local procedures. Maybe the local instructions requires the driver to leave the maneouvering area via a specific route or in a certain direction. In such cases, that would be written in the local instructions. They are called local because they are only valid at that specific airport.