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Howdy. The ’10’ is the priority of the filter (normal) and ‘3’ is the number of arguments the oci_login_redirect() function takes.

https://codex.wordpress.org/Plugin_API#Filters

If you take a look at the apply_filters() call that is in wp-login.php it sends along 3 params. The second param is not so obvious:

isset( $_REQUEST['redirect_to'] ) ? $_REQUEST['redirect_to'] : ''

Read it as:

‘if the redirect_to is set to something then use that as the 2nd param and if it isn’t set then pass along nothing as the 2nd param’.

Just in case we want to do something fancy in the filter that has to do with where the redirection was supposed to go in the first place.