The documentation here tells the difference:
d - is day of the month
w - is day of the week
Now for your purpose / dateadd () function - the interval parameter only indicates what unit of time
you are adding to your initial date [10 days, months, years etc]
So for this function: d, w, and even y will be identical, because they are only telling the function "add x days"
For other functions there is a difference between the two