Use this to filter the CSS classes added to the fields on the checkout page. This is normally used to handle error highlighting with the “pmpro_error” class.
This returns the CSS classes and the current the current field name to be applied to.

apply_filters("pmpro_field_classes", string $classes, string $field);

Parameters

$classes

string

$field

string

Source

File: https://github.com/strangerstudios/paid-memberships-pro/blob/master/includes/functions.php