Host-to-Host is an automated, two-way data transfer service for high-volume online banking that gives you direct, anytime access to Business Online from various applications. Host-to-Host is a direct system-to-system integration between your ERP system and the bank, and with no user intermediary involved.
A host-to-host connection is a two way method of connecting direct to specific banks. How those work in specific then depends on the bank i.e. the bank defines the format messages are exchanged, rather than it be a predefined protocol. With some banks a host-to-host connection is needed for each country where the company is operating.