Interfaces and protocols
SOAP
In short and in plain language
SOAP is an older way for programs to share data.
Many stock and accounting programs still offer it. Their connections have grown over many years.
It has strict rules. That makes it more work to adapt.
It is a good choice when the other side offers nothing else.
Definition
SOAP is an older, XML-based protocol for system interfaces. Many ERP and accounting systems still offer it because their interfaces have grown over the years. It is more strictly typed than REST and therefore more work to adapt, but it is a solid choice when the other side offers no alternative format.