Why is this important? Nearly every reenactment group uses a military unit designation to identify itself. This designation is part of the "brand" of a reenactment unit. Facebook pages and web sites will bear these designations, as will camp or barracks signage at reenactment events. An incorrectly rendered designation not only can be embarrassing to a unit trying its best, but it is an authenticity issue, as well.
The good news is that identifying the correct unit designation, and abbreviating it correctly, is not terribly difficult. The Wehrmacht was a force of millions of men, in thousands and thousands of different units, so possible unit designations and abbreviations are nearly infinite. But nearly all reenactment groups portray the same types of units (largely ground combat units, mostly Heer and Waffen-SS) and the designations for these typical units are easy to identify and describe.
Let's start at the beginning, at the most basic level. One soldier. In most units, he is in a squad- a Gruppe. Squads are grouped usually into a platoon- a Zug. These are generally really not important from a unit designation perspective. The next level, though, is important. A number of platoons would form a Kompanie (in Artillerie units, a Batterie; in Kavellerie units, a Schwadron). The Kompanie is extremely important for the reenactor. In nearly all units, this Kompanie designation would be in every soldier's Soldbuch, the identity document every soldier carried. The soldier would know what Kompanie he was in. The Kompanie is designated by an Arabic numeral, for example: 3. Kompanie, abbreviated 3. Kp.
The Kompanien (plural of Kompanie) would be grouped into a Bataillon. This is designated by a Roman numeral: I. Bataillon, abbreviated I. Btl. For nearly all unit designations, the Bataillon designation is not only unnecessary, but altogether superfluous and would not be used. The reason for this is that the Kompanien were numbered sequentially in a Regiment and assigned in a regular way to a Bataillon. In a regular Infanterie-Regiment, generally speaking, 1.-4. Kp. would be I. Bataillon, 5.-8. Kp. would be II. Bataillon, 9.-12. Kp. would be III. Bataillon. So if you are in 1. Kompanie you would never need to indicate that you were in I. Bataillon because 1. Kp. is always in I. Btl. and there is no 1. Kp. in any other Bataillon in the Regiment. In general, the only time Bataillon needs to be included in a unit designation is when the unit being portrayed is a Bataillon staff. Reenactors get this wrong very often.
With Bataillon being superfluous, the next thing that is important after Kompanie is the designation of the Regiment, Abteilung, or, sometimes, independent Bataillon (depending on unit type). This is usually a numbered unit and the number comes after the unit type: always Artillerie-Regiment 3 or Pionier-Bataillon 3, never 3. Artillerie-Regiment or 3. Pionier-Bataillon. This part of the designation is crucial. It is most important. This part of the designation is, in virtually all cases, what determines what unit insignia a reenactor wears. The Waffenfarbe branch color worn on insignia is determined by the Regiment or Abteilung. Before the use of unit numbers on insignia was abolished, the number of the Regiment or Abteilung is what would be worn on the shoulder straps.
The next level up from that is the Division. This aspect of structure is of almost no importance at all unless you are portraying a Divisional staff. The reason for this is that the number of the Regiment or Abteilung is unique and only used once in the entire armed forces. There was only one Panzer-Regiment 6, for example. There would be no need to say "3. Panzer-Division, Regiment 6" because there was only one Panzer-Regiment 6 in the whole Army and it happened to be in 3. Panzer-Division. In any other Panzer-Division, each Regiment also had its own unique number, used only once. The designation of the Regiment or Abteilung makes the use of the Division designation totally superfluous. It would not be included because it didn't matter. A designation like "3. Panzer Div. Nachrichten-Abteilung" for a signals unit in a Panzer division is totally wrong. It is missing the important unique number designation (39, in this particular case) and it is including the Division which is superfluous and shouldn't be included. The correct designation for this unit is, simply: Nachrichten-Abteilung 39. The fact that it happens to be attached to a Panzer-Division, not some other kind of Division, is unimportant and has no impact on the unit designation.
With that out of the way, we can get down to the nitty-gritty of what the unit designation looks like and how it can be abbreviated. The designation of the unit we portray is:
3. Kompanie, Sicherungs-Regiment 195
3. Kp., Sich.-Rgt. 195
or more simply- and more typically:
3./Sich. Rgt. 195
Do you see the slash in that second abbreviation? That is something that seems to trip people up all the time. It's not the Division, not the Regiment. The number before the slash is the Kompanie, or the Bataillon. How to know which is which? Kompanie is Arabic numeral, Bataillon is Roman numeral. 3./Sich.-Rgt. 195 was one Kompanie in I./Sich.-Rgt. 195. Please note also that if you are using the abbreviation "Kp." for Kompanie, you don't also use the slash. 3. Kp./Sich.-Rgt. 195 would not be correct, even if it is not inconceivable that even some wartime Germans might have made that mistake. And don't use the slash in any other way. That's basically it. A Kompanie in a Pionier unit might be 2./Pionier-Bataillon 22, a Schwadron in an Aufklärung unit could be 2./Aufklärungs-Abteilung 207. Note that the umlaut is a really important part of spelling, it's not Aufklarung, or Jager. If you can't type the umlaut you have to put an "e" after the vowel that is supposed to have the umlaut: Jaeger.