From ErfWiki

[edit] Proposed Canon

A Column is a name for multiple adjacent hexes of Stacks from the same side or its alliances, usually traveling together to assault or defend a specific location.

[edit] Speculation

Columns seem to be a reaction to a maximum troop capacity per hex. It isn't clear whether there are a maximum number of Units per hex, or Stacks per hex, or whether there is a bonus up to some maximum number of units, as with the 8-unit Stack bonus limit.