When you create a new custom field on a standard object or a custom object, by default there is some FLS. (I mean for some profiles the Visible checkbox is checked and for some it is not.) How is that determined? It does not seem to match the object settings on the profile. Hence the question. Also for some fields Visible is checked on all profiles and I cannot even adjust it , as in I cannot uncheck the checkbox for any profiles. Can some one explain how FLS works?
When you create a new field it is generally available to all Standard profile exclude the community or chatter profiles but if you want you can give permission to them or can change the permission.
Also when you create Required field and Master detail relationship then this field is available for all profiles and you cannot remove this(due to required thing.) You can find more information here and here.