Due to Obedience, commands normally must be followed. Some units can disobey if doing so is in the best interest of the Side, or if a higher authority issued conflicting orders; however, disobeying an order can cause a unit to disband.
New orders can amend previous orders. WB2014 Duke Forecastle - Part 2
Standing orders are followed until further notice. For example, a unit may have a standing order to trigger a crap golem explosion if it ever finds itself outnumbered,Erf-b3-p200 or unled garrison units may be given standing orders never to engage a specific target.Erf-b3-p215
Silent orders via natural Thinkamancy might be Ruler-only, because Parson wasn't able to order Lilith to make contact when she was causing Contract violations inside Charlescomm. Parson had to contact Stanley, who was then able to send the order to Lilith.
This could simply be a Parson-specific limit, not applicable to other Chief Warlords, who may have more experience/practice with giving silent orders, or who may have popped with a natural ability that Parson lacks.