DB-ALLe stores meteorological values in a database. This database can be optionally shared or accessed using the network.
You contact DB-ALLe by creating a connection, and you work in DB-ALLe using one or more sessions. A connection is an established link between your program and DB-ALLe, and you usually open it at the beginning of your work and close it at the end.
Within a connection you can create many working sessions. This is very useful because it allows to do different things at the same time, like reading a set of values while writing computed results.
You can also set some safety features on sessions: for example, when you create a session for reading values you can set it to disable all writes, which helps you catch some programming mistakes.
You refer to the connection and the sessions using handles. A handle is just an integer value that you use when you need to refer to something that is not otherwise representable in any Fortran data type.
When you create a connection or a session, DB-ALLe will give you the integer handle that you will later use to refer to it.