The condition is met if the current request is a WP AJAX request and the action is the selected AJAX action. You can use Exact, Partial or Regex mode.
There is an autocomplete option, which contains all registered AJAX actions, but you can use Partial or Regex mode as well.
Exact mode
If you use the exact mode, the condition is met only if the current AJAX action is exactly the same as the action specified. 
Partial mode
In partial mode the condition is met, if  the current AJAX action contains the specified action (part). For example if you woocommerce_ it will match for all WooCommerce AJAX requests (like woocommerce_add_to_cart, woocommerce_apply_coupon, etc.)
Regex mode
You can use regular expression here. The condition is met if he current AJAX action matches with the specified regular expression. For example if you specify add-(\w)_tag, it will match for add-post_tag and add-product_tag as well.