Do you know why some Doms wants to be addressed as “Sir” and others as “Master?”
Titles are not always an indicator of the type of relationship(s) they are in. As a result, I recommend that you always ask people that you are interested in why they have adopted a certain label or title and what it means to them. It is best not to assume that there is anything more to the title than personal comfort with it.
However, in the case of "Master" and "Sir" it is often the case that the title of "Master" is more often adopted within the Master/slave dynamic. The title of "Sir" is more common within a Dom/sub dynamic. This is not a hard and fast rule. There are many other titles that people choose to adopt such as "Daddy" and "Lord." It is also worth noting that even though a title might seems to indicate gender, that is not always the case. There are females who adopt the title of "Sir" or "Daddy."
Just like the titles themselves, what these words mean to any given person or relationship can greatly differ. There are no absolute rules. Hence my advice to talk to people about their preferences so that you can get the best understanding of what it means to the person. After all, these are human relationships. Just as no two marriages will be exactly the same, no two kink relationships are the same.