Proposed Canon
Scouts are units that can go behind enemy lines and observe enemy movements and numbers, notably without needing a Warlord to be stacked with them to avoid them from automatically attacking upon encountering enemy units like un-led infantry normally do. They seem able to choose when to engage enemy units, if at all.
It's as yet unknown if a scout is a type of unit or a Special. Scouts could be like Knights, advanced infantry without a unique special. Or they may be similar to Digger class Infantry, who have the Digging special. A scout may be a unit with the "Scouting" special. Further confusing matters, there appear to be non-scout units that can fulfill the role and go scouting, such as Warlords.
There are several types of unit that can fulfill the role of scout, including Men, Natural Allies and Wildlife. Haffaton used men as scouts, outfitting some with very good Veil capable Raiment to hide in plain sight. The Royal Crown Coalition used Marbits as tunnel scouts during the siege of Gobwin Knob. Transylvito Warlords are able to use multiple Doombats and see through their eyes as a form of Natural Magic, possibly Lookamancy or Thinkamancy. Charlie's Archons are also capable of scouting, being particularly good thanks to their Foolamancy.
It's not been explicitly established how the scouting unit relays the intelligence they gather. It may be through Natural Thinkamancy that gets relayed to their Ruler or Chief Warlord, or they may be given a Message hat and parchment so they can give more detailed reports.
 Known Scouts
The following units have performed in the role of scout or are scouts themselves.
Rangers may be either a subtype of scout, or a related class capable of tracking and hiding.
Scouting is a means of gaining information instead of or in conjunction with Lookamancers.
A scouting unit is generally posessed of a high Move, is cheap (and therefore expendable), or, ideally, both.
To be of use, a scouting unit must have a means of conveying the information it gathers to its superiors.
Some methods used:
- Thinkagram, as offered by Charlie's archons
- Message Hats
- Passing notes from hex to hex along a caravan.Erf-b1-p052-p14
Barring all of these, messages can be hand delivered by troops with the Move available to reach their Commander.
 Taking Turn With the Enemy
It's possible that the presence of a solo scout does not alter the turn order in a battlespace. Normally when units from two non allied sides are in the same battlespace the unit(s) taking turn later on become aware there will be an encounter when their turn doesn't start at dawn.IPTSF_Text_27. However when Goodminton forces sallied to fight HaffatonIPTSF_Text_5 they weren't expecting any encounters (evidenced by singing bawdy songs rather than looking for enemies). Later they spotted and croaked a Haffaton scout. This could mean that scouts can take their turn at the same time as an enemy side similar to how prisoners take their turn at the same time as the side which captured them.
This would be a valuable ability to make sure scouts aren't discovered by hiding their presence from the Natural Predictamancy of Battlespace Determinations. Though considering Doombats, the most widely seen scout, do take their turn with the rest of their side, this ability may only be available to men.
An alternate explanation for this relies on the above instance being due to carelessness on the part of Goodminton forces, or an earlier engagement making them think there wouldn't be any more in the same day. Meaning the presence of scouts in a battlespace always tips off the spied on side. This would coincide with the instance in LIAB_Text_18 when Archons explicitly end turn before Haggar and one is captured in enemy action. In that instance though, Archons are Knights, not scouts.
If Parson's plan in LIAB_Epilogue_17 to make a Stealth Golem scout through a caster link between Jack and Ace relies on keeping Charlie in the dark about the Golem's presence nearby it would have to avoid taking its turn after Charlescomm. Otherwise he would know enemy forces are nearby.