BALANCE_SIP_HEADER specifies the header name of balance value, which is sent inside SIP message.
See details here: Display of Balance (Parse SIP headers).
The balance is encoded inside SIP response message to REGISTER and INVITE requests, for example:
SIP/2.0 200 OK
CSeq: 2 REGISTER
From: ...
To: ...
Balance-amount: 10.25
In above example, SIP message contains a custom header "Balance-amount", which is added by billing system. It has value 10.25
In above example, BALANCE_SIP_HEADER inside installer-brand.nsi should have following value:
!define BALANCE_SIP_HEADER "Balance-amount"