From: Connecticut, USA
The game calculates the supply value to decide what is a "legal" move and what is not, and how much supplies get through. Bottom line is just like real life, the more difficult you make it to get supplies, either because you are cut off, or farther from a base, and/or one or more hexes off a highway or road, you can expect fewer movement options and less supply. The best bet is to look in the manual for specifics. I think 14.3 on page 143 may answer most of your questions. For this purpose a jungle hex is considered "cross country", though jungle terrain does affect movement rate and combat. From the manual:
14.3 GROUND UNIT SUPPLY
When tracing a supply path for movement of ground units or overland movement of supplies, a supply value is generated. This is determined by tracing a path from the base transferring supplies or the ground unit moving to the destination. This supply value determines whether the move is legal, and how much of the supplies are used up during the move. The supply value of a move is calculated by subtracting the following from 100 (900 if a ground unit moves):
• 2 for each hex moved along a rail/highway
• 5 for each hex moved along a road
• 25 for each hex moved along a trail
• 50 for each hex moved cross country
To order a ground unit move to a specific hex, the path of the move must generate a positive supply value to constitute a valid supply path. Ground units will move along a path that provides the best possible supply route from their current hex to their destination. For automatic supply movement, the supply value must be greater than 0. The greater the supply value, the more often supplies will be moved, and fewer supplies will be expended during the move.
Example: A unit moving from hex 54, 87 (Lae) to 54, 89 generates a supply value of 825 (1 hex of trail movement and 1 cross-country, 900-25-50=825). Since the value is positive, it is a valid move. Note that if there were enemy units in hex 54, 87, a movement plot to 54, 89 would not be allowed as it is not a friendly base. If supplies were being automatically moved from 54, 87 to 54, 89, it would return a supply value of 25 (100-25-50) which would allow some supplies to transfer. However this is a low value so supplies would not be transferred every day, and many supplies would be expended during the transfer.