As long as Field Engineers remain within 3" of other friendly Infantry units with Iron Discipline, they have Ward save (4+) against enemy spells and missile attacks, and may Stand & Shoot against enemies charging their "parent" unit using the normal rules. They are otherwise completely normal units for the purposes of Movement and for determining Victory Points.
All units of Field Engineers must deploy at the same time, just like War Machines. Unlike other War Machines, the Dragon's Breath and Hand Mortar may march.