Filters the HTML content shown when the submit button is clicked on the checkout page. Passes a text string by default, but can be used to pass new HTML code as well.

apply_filters("pmpro_processing_message", __("Processing...", "pmpro"));

Parameters

string ( __(“Processing…”, “pmpro”) )

Source

File: https://github.com/strangerstudios/paid-memberships-pro/blob/master/pages/checkout.php