I created a new custom field for one of my projects along with field configuration and field configuration scheme, at the end - I noticed few other projects were effected by. may I know why?
Did you define the field as global, and put it on screens shared with other projects? Do the existing field configurations all hide the field after you edit them?
Generally adding a field is a global thing, you have to remember to limit the context if you're limiting where they are used that way. The other ways to have them not appear in other projects is by not adding them to screens the other projects are using, and setting the field to hidden in the other projects field configs (the default for a new field is visible)