Your question is explicitly answered in section 18 of the rules, under the Leadership cards section:
Units in “adjacent linked hexes” may be in different sections of the battlefield, as long as each ordered unit is adjacent to at least one other
ordered unit, and at least one of the ordered units is adjacent to the designated leader. In this way, a Leadership order enables you to order a “chain” or “wing” of units to move and/or battle in a coordinated fashion
in addition, there is a diagram below the rules text to further clarify.