Line Command: Order a group of foot units. The group must be in adjacent linked hexes (which may span two or three sections). Each unit may move no more than one hex and then engage in Ranged Fire or Close Combat if eligible. Units may Momentum Advance after successful Close Combat and may be eligible for bonus Close Combat. If you do not have any foot units, order 1 unit of your choice. (4 cards)
NOTE: Ordered units do not have to move to engage in combat.
Can a Line Command order units in different sections?
Yes.
When a Line Command card is played, can some of the units not move and just fire?
Yes. Units that are part of a line command are not required to move. Units that are armed with missile weapons that do not move or only move 1 hex may fire.
Does the "Line Command" group need to remain in adjacent linked hexes after each unit moves? Or can the adjacent units from the "line command" group split into two or three separate groups after they move one hex each?
There is no rule to prevent this so you are allowed to split units.
Is it possible to use Line Command card to order 1 single unit? (because rules refer to a group of foot units).
Of course you can use Line Command to order a single unit. There is no restriction to the group size: one foot unit is a line of size "1".