Since the 1930s, the sky is officially divided into 88 constellations. At that time, the boundaries between the constellations were clearly defined, too: namely by the coordinates of sky points that, when connected with their respective neighbours, create the boundaries.
Every constellation has a Latin name and a three letter abbreviation.
For example “Orion – Ori”, or “Ursa Major – UMa” (Great Bear), or
“Hydrus – Hyi” (Male Water Snake). This abbreviation plays a big
role in PP3 because it's the only way to denote a constellation.
You must use only uppercase letters in the abbreviation: ORI,
UMA, HYI.
One constellation, the snake or “Serpens”, is divided into two parts,
“Serpens Caput” (Head) and “Serpens Cauda” (Tail). They are
abbreviated SER1 and SER2 in PP3. If you say
SER only, you mean both parts.