So most recruitment rules are based around on reforms, general level, and general class. A reform will first unlock the unit for recruitment across the faction. However, in order to unlock that unit for recruitment by a specific general, they both have to have the appropriate level (mid tier at level 4 and high tier at level 6) and the appropriate class (most cases, does the color match? exception is commanders, who can recruit most mid tier).

The main exceptions are Palace Guard, Imperial, and Dragon units, which are available to everyone in your faction once you unlock the appropriate reform or reach emperor status.

To address the example, Archers are recruitable by Strategists (blue) and Commanders (yellow) at level 4 once you have the right reform.