When an Institution is connected, Exirio may not always receive the full transaction history in relation to the various Positions included in a Securities Portfolio or Wallet. This may cause several inconsistencies, like a mismatch in Units Balance: if you bought three Apple shares many years ago and sold two last month - and we receive only data for last year - we will have Transactions that generate a negative balance of two units, against a real balance of one unit. 
When this happens, i.e. when we are limited by either our connection partners or by the Institution you have connected in how far back in time we can retrieve data, we add one initial Balancing Transaction. This Transaction is added to make numbers match, and it's meant to "summarize" all your activity prior to the earliest date we have data for. It is up to you whether to leave it how it is, or manually replace it with the actual historical Transactions. In the example of Apple shares, we would add one Balancing Buy Transaction for three shares.