ALMA is an acronym meaning Arena Library MAnagement.
ALMA is a set of services used by Arena to communicate with the Library Management System (LMS) for information retrieval and update.
The root element of ALMA is ‘AlmaMessage'. An instance of an ALMA service XML message must contain a single ‘AlmaMessage' element, which in turn contains a single child element taken from the list in ALMA Services.
ALMA services messages are in pairs, each service consisting of a request and a response message. Every response message contains a common status element indicating whether the request was successful, with an error message to indicate the reason for failure.
A number of ALMA service requests contain ‘branches' and ‘organisation' attributes that are used as filters. When configuring Arena, you may choose to configure the site to show a subset of the LIS by setting organisation or branch filters. The values are passed to the LIS, and are handled according to the LIS business logic. Implementation of the add/change messages is not consistent. Some messages only return the actual item (debt, loan, reservation, etc.); others return a list of items. The branch and organization filters are used when the message returns a list.
You find all you need to know about the ALMA Services in Arena ALMA Services here (scroll down on that page).