To Export Out Of Available For The Destination
To Export Out Of Available For The Destinations shows the number of customers out of segmented customers that have mandatory attributes (if defined) present.
For example, below the number of exported customers is 120, while segmented 121. This means that out of 121 segmented customers, 120 have a mandatory attribute (that has been set by an administrator) present.
"To export out of available for the destination box" shows "120 out of 890". This means that 890 customers in total have mandatory attribute present and out of this number exported will be 120 customers.
To check which attributes are defined as mandatory and how to change them, please refer to this article.