Polycom MGC Administrator Administrator's Manual page 134

Chapter 3 - Defining Network Services
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c. Click OK to add this user (UA) to the authenticated users list, and
return to the Security dialog box.
16. Click Next.
The Span dialog box opens.
This dialog box lists the currently defined spans to be used with the
defined Network Service. This dialog box is used to define the cards to
which the network, whose properties are defined in the Network Service,
is connected. In this dialog box you define the IP cards that can be used
to handle incoming and outgoing calls from/to the gatekeeper and/or SIP
server.
A span defines the card's parameters and network settings; it is set
separately for each IP card as each IP card has a unique IP address and
can also have different capabilities than other installed IP cards.
The Circuit ID is the connection between the span and the card; it
identifies the specific span with a numeric identification, which you use