A house system is a method for calculating or assigning the twelve house boundaries in an astrological chart. Systems can use equal zodiac sectors, whole signs, or divisions derived from time and space.
The method matters because different systems can place the same planet in different houses while leaving its celestial longitude unchanged. Some systems align particular cusps with the Ascendant and Midheaven differently.
No house result should be presented without naming the system. A borderline placement deserves its exact degree and sensitivity, not a false claim that every tradition agrees on one label.