The notional amount is the face value of an instrument and indicates how much of a particular security an investor has. This is to be contrasted to the market value, which indicates how much that security could be sold for in the market.
EU Regulation with regard to regulatory technical standards on indirect clearing arrangements, the clearing obligation, the public register, access to a trading venue, non-financial counterparties, and risk mitigation techniques for OTC derivatives contracts not cleared by a CCP:
"To further mitigate risks, portfolio reconciliation enables each counterparty to undertake a comprehensive review of a portfolio of transactions as seen by its counterparty in order to promptly identify any misunderstandings of key transaction terms. Such terms should include the valuation of each transaction and may also include other relevant details such as the effective date, the scheduled maturity date, any payment or settlement dates, the notional value of the contract and currency of the transaction, the underlying instrument, the position of the counterparties, the business day convention and any relevant fixed or floating rates of the OTC derivative contract."