I've been working on something Sarmatian-y to break up the slew of KWHSS posts, but their's a lot of information I should discuss before I actually get to it which is not directly related to Sarmatians. Consider this post 1 of 2 leading up to it.
I realized that I've been discussing devices and badges without actually explaining what or why they are, which might be confusing to someone not familiar with the topic. Devices and badges are collectively called "armoury".
In the SCA, you don't actually get to call your device "arms" until you've been given an award of arms level award. This piece of armoury is what you likely envision when you think of designs on shields and banners. When registered in the SCA, your arms are designed in a shield shape like this:
You don't need to register a device to use it in the SCA, but doing so ensures that no one can register a device similar enough to yours that the two could be confused from 10-20 feet away. Another benefit of registering is that if you yourself are not a herald, you'll have the collective opinions of the SCA College of Arms weighing in on your device to see if it conforms to period style.
There is a caveat to that last statement- Just because a design is registrable via SCA rules does not mean it's good period style (and you might come up with good period style on your own without registering). You'll likely have some heralds commenting that doing such-and-such to it would make it more period, but you won't be rejected purely for that reason. The SCA is its own heraldic jurisdiction. Our rules, though based on what our group has gleaned from period rolls of arms, do give some leeway for avenues of design which those in period might not have used simply because it wasn't aesthetically pleasing to them. (See this post for more details.)
Aesthetics change over time and the SCA Heraldry Rules for Submission allow for this creative modification of a very medieval thing (one reason for the "creative" in Society for Creative Anachronism). Some are more diehard and want very typically period armoury, but if you compare a period roll of arms and an SCA roll of arms, you'll notice that common themes are very different between the two. Especially since our understanding of period style has changed over the years. You'll see some things that used to be allowed which no longer are and sometimes vice-versa.
Period style or not aside, devices/arms in the SCA serve the same purpose as those in period- they mean "This is me." Now, depending on what stretch of period time you're talking about "me" might actually mean "me", or it could also mean "my family", "my province", or "my kingdom".
The original purpose of arms was to identify friend vs. foe on the field of battle. Identification is much quicker that way than taking to the time to assess his armor- especially given that the styles could be similar. Family members might have similar devices only barely different by something called a "cadency step" (like changing the color, adding a charge, etc...). Later on, specific arms were shared by entire families, which is more common today (try looking up your family crest, mon, or other related symbols based on paternal ethnicity). Royal arms became symbols of kingdoms.
Badges serve a similar purpose but often have a slightly different nuance- "This is mine." Much like maker's marks, branding on animals, or the initials your mother may have written on the tags of your clothing, it indicates that something belongs to you. In the SCA, badges are also used as a sort of "clan device". Households in the SCA are groups of people who share a common interest, common location, etc... As non-person entities not recognized as official SCA groups (like shires or baronies), they cannot register devices. They can, however, register badges, which are just like devices except they're registered on a square instead of a shield and can have a tinctureless field (background). That tinctureless field means you can put just the ordinary(ies) on an object without needing the entire thing to be a square shape. Badges also tend to be much simpler than most SCA devices/arms.
People often register badges which are similar to their devices. Ex: John Doe's device might have a gules (red) field with an Or (yellow) lion's head erased (like it was ripped off) within a bordure (border) which is also Or. His badge might be just the Or lion's head erased. He displays his device on his shield and on a banner by his pavilion at camp and his badge as a small detail on his possessions.
They could also be something completely different to delineate between "this is me" and "this is what I do". Ex: Jane Doe's device has an azure (blue field with an argent (white) bend (very thick diagonal line) between two peacocks also in argent because she likes peacocks and thinks bends look cool. She might be a blacksmith, so her badge, which she could put on everything she smiths, might be an azure anvil. If someone buys a helmet they're fond of and that anvil is indented into it, they could look through the SCA armorial (or ask around) to find out who made it so they could commission a breastplate from her.
The Sarmatians had their own brand of heraldry which looked nothing like the Western type we're familiar with, but to discuss that, I should first delve into regional styles of heraldry.