The term Commander is used in two separate ways. All units with Leadership, including Commanders, Warlords, and Chief Warlords, are periodically called Commander by their associates as a title. The actual rank of Commander indicates that a unit has the Leadership Natural Ability, but no confers no Leadership Bonus to units under its command; whereas, Warlords and Chief Warlords confer a Leadership Bonus to units in their stack and in other cases as well.
Contradiction in Terminology
It seems that "Commander" is a more general term - Commanders have an ability to lead other units in combat (not necessarily deriving from Leadership natural ability) while Warlords are Commanders who have Leadership ability and thus give units under their command a Leadership bonus. It should be noted that in some special cases Casters can give a bonus to certain units under their command (known examples are Golems led by a Dirtamancer and uncroaked led by a Croakamancer)Erf-b1-p084a.
Duty is one of the three effects of Natural Thinkamancy (besides Obedience and Loyalty) that a Ruler exerts on all his subjects. Duty affects Commanders only. Has greater effect on Warlords, and greatest on Chief Warlords. Duty requires Commanders to use their own initiative in the service of the Ruler. Another effect of Duty is that Commanders cannot withhold information and cannot conspire against the Ruler.Erf-b1-p084a
Use as a Title
Word "Commander" is used as a title to address Warlords and, occasionally, Casters. In this sense words "Warlord" and "Commander" seem to be used interchangeably, although Commander seems to express more respect.
Stanley addresses Wanda as CommanderErf-b1-p017, Wanda addresses Parson as Commander Erf-b1-p019, Jillian addresses Webinar as CommanderErf-b1-p050, Webinar addresses Jillian as WarlordErf-b1-p050, Ansom addresses Jillian almost exclusively as CommanderErf-b1-p056Erf-b1-p056Erf-b1-p073Erf-b1-p080, Jaclyn addresses Jillian as Commander Erf-b1-p066, another archon addresses Jillian as WarlordErf-b1-p069.