Portfolio exogenous bookings are transactions, which are cross-portfolio.
This line is only visible when you select the Performance Report for a portfolio or a securities account, since cross-portfolio transactions are only possible for these objects.
For example, if a equity is purchased into securities account A under portfolio A, but the settlement account for this purchase belongs to portfolio B under the same owner, then you have a portfolio-exogenous flow on both portfolios. These are taken into account when calculating the performance.
For example, if you evaluate at securities account level, all account postings are displayed as "portfolio exogenous", since the account is not included at securities account level.
The template "Cross-portfolio transactions", which you can find in the workspace of securities accounts and portfolios on the worksheet "Transactions", lists all these transactions.