Capacities in the intraday system are provided by the TSOs. The cross-border capacities are handled implicitly, that means local order book views take into account capacities that are allocated by the different TSOs on each border.
Cross-border trades will only be possible if there is enough allocated capacity between the areas.
Intraday capacities are determined by the relevant TSOs after the flow results of the day-ahead auction. The exact timing of capacity allocation varies and depends both, on operational procedures and individual agreements between the TSOs on the different borders.
Intraday capacities are updated automatically, depending on the volume and direction of intraday trades. Capacities can further be changed directly by the TSOs as a result of operational issues, e.g. an outage on a transmission line.
As in the day-ahead auction ramping restrictions apply in intraday. As a result the entire remaining capacity after the day-ahead auction may not be available for the market on ramped connections. A full description of ramping restrictions can be found here.