Hi,
The term “sifu” (“shifu”) isn’t different, and does not vary from martial art or branch, or sub-system. It is what it is.
The proper term “sifu”, in a martial or religious context, combines the character for teacher (si/shi) with the character for father (fu/fu). It is a relational term, not a rank, title, etc. The person who taught you is your sifu, no one else. Just like your father is “father” to you, but your friend’s father isn’t “father” to you, just to him. The term, like the relationship, is personal (and can be profound).
Furthermore, there is no requirement for the term “sifu” other than the person being your martial teacher. They can have experience or not, skill or not, a school or not, be qualified or not–as long as they agree to take you as a student (todai/toudi or daijee/dizi) and you agree to take them as your teacher, they’re your sifu. (Just like anyone, from a 14 year old, wet-behind the ears kid to a mature 50 year old man can have a child and become a father).
Note: There is another term that sounds identical that often causes confusion: a sifu/shifu with different characters is teacher-teacher (not teacher-father) and is an honorific or term of respect used for anyone who has special skill. In China, this can be used for martial artists, cooks, even taxi drivers (since in the early days driving was considered a skillful profession). Anyone skilled at a trade can be referred to by others as a sifu (though, at least properly, you would never find someone calling themself sifu).
In modern times, some have used it as a title of rank, or a degree to be obtained, as some have added colored sashes or other organizational/commerical trappings. While this might be necessary for them, it does not extend beyond them, and every individual and other organizations might have very different ideas about the requirements or even the usage itself.
Rgds,
RR