Path to a file containing the accounts to use for external SIP bridging, organised by provider, in JSON format.
Here is a template of what should be in this file:
{
 "schemaVersion": 2,
 "providers": [
  {
   "name": "<user-friendly provider name for CLI output>",
   "accountPool": "<name of an account pool described below>",
   "triggerCondition": {
    "strategy": "<MatchRegex|Always>"
    "pattern": "<MatchRegexParam: regex>"
   },
   "accountToUse": {
    "strategy": "FindInPool|Random",
    "by": "<FindInPoolParam: alias|uri>",
    "source": "<FindInPoolParam: {from}|{to}|{sip:{incoming.to.user}@{account.sipIdentity.hostport}{incoming.to.uriParameters}}>"
   },
   "onAccountNotFound": "nextProvider|decline",
   "outgoingInvite": {
    "to": "<{account.alias}|sip:{incoming.to.user}@{account.sipIdentity.hostport}{incoming.to.uriParameters}>",
    "from": "<optional: {account.sipIdentity}|{sip:{incoming.from.user}@{account.sipIdentity.hostport}{incoming.from.uriParameters}>",
    "outboundProxy": "<optional: sip:flexisip.example.org;transport=tcp>",
    "enableAvpf": <optional: true|false>,
    "mediaEncryption": "<optional: zrtp|sdes|dtls-srtp|none>"
   }
  }
 ],
 "accountPools": {
  "<name of account pool>": {
   "outboundProxy": "<sip:some.provider.example.com;transport=tls>",
   "registrationRequired": <true,false>,
   "registrationThrottlingRateMs": <optional: number>,
   "unregisterOnServerShutdown": <optional: true|false>,
   "maxCallsPerLine": <number>,
   "loader": {
    "dbBackend": "<mysql|sqlite3>",
    "initQuery": "<SQL query>"
    "updateQuery": "<SQL query>",
    "connection": "<db=sip_accounts user='flexisip-b2bua' password='secret' host=db.example.org>"
   }
  },
  "<name of another account pool>": {
   "outboundProxy": "<sip:some.provider.example.com;transport=tls>",
   "registrationRequired": <true,false>,
   "registrationThrottlingRateMs": <optional: number>,
   "unregisterOnServerShutdown": <optional: true|false>,
   "maxCallsPerLine": <number>,
   "loader": [
    {
     "uri": "<sip:account1@some.provider.example.com>",
     "userid": "<optional: (e.g. an API key)>"
     "secretType": "<registrationRequiredParam: ha1|clrtxt>",
     "secret": "<registrationRequiredParam: password or API token>"
     "realm": "<optional: realm.some.provider.example.com>"
     "alias": "<optional: sip:anotherAccount1@some.provider.example.com>
     "outboundProxy": "<optional: sip:another.providerOverridingPreviousOne.example.com;transport=tls>",
    }
   ]
  }
 }
}
Full documentation is available here: https://wiki.linphone.org/xwiki/wiki/public/view/Flexisip/Configuration/Back-to-back%20User%20Agent%20%28b2bua%29/SIP%20Bridge/#sip-bridge