You can manually include and exclude members of a segment as per this functionality:

https://learn.microsoft.com/en-us/dynamics365/customer-insights/journeys/real-time-marketing-include-exclude

When using this, it turned out that there is a limit of 100 members that can be added, which is not documented.

This feature is very useful, especially when doing cleanup work after journeys that have been wrongly designed and corrected, by identifying the contacts that need resends and do that manually by adding them to a resend segment manually. Also the selection of contacts could be improved. It is possible to use views to pick from, but a select all from the viw function would also be useful, instead of picking every single member from the list. That is a lot of clicks if you want to select a couple of hundred contacts.