In general, incidental costs/ expenses, or incidentals, refer to any costs or expenses that relate to performance (of a fund, instrument, portfolio, etc.)

For a financial instrument, for example, incidental costs are the costs and charges that are tied to events occurring, or to occur, during the holding period of a financial instrument, giving rise to performance fees.

For a fund, incidental costs may refer to transaction costs that are incurred by a fund manager whilst running the fund and its investments, etc.