Error field of new Meteor.Error & legacy problems of built-in Meteor functions

In document,

For legacy reasons, some built-in Meteor functions such as check throw errors with a number in this field

What is it meaninig? If I put a Number in “error” field as “throw new Meteor.Error(403, “This is 403 Eror”)”, check() might throw an error ?

And what are there legacy problems associated with built-in Meteor functions ?