APERAK stands for APplication ERror and AcKnowledgement message.
It is an EDI (Electronic Data Interchange) message type, used to send an acknowledgement back to a sender who has sent, for example, an invoice. It tells the sender whether the original message (eg invoice) was OK or not, and if not OK, what was wrong with it, eg invalid amount, incorrect item, value etc.
It should always be shown in CAPITAL LETTERS (see 1st ref!).
It is not a program. It defines the contents of a text message sent electonically (but not by email), but via specialised EDI services (I know, because I used to work for such a service).
For an example of Swedish use, see 2nd ref.