An Interrrail pass, no matter what length, is only valid for 2 days in your home country.

With the exception of that rule you are free to travel when and where you please.

And these 2 days-named OUT and INbound-are meant to go to the border as the pass is for travel out of own country.

Irrespective of the name you are not obliged to leave or return to your own country with these days. They are there to assist your Interrail experience and can be used as you want, if at all.

There is absolutely nothing to stop you using them to travel to or from airports or ferry terminals, nor are they limited to trains crossing a border.

They do not have to be continuous with journeys outside your own country, merely within the validity period of your pass.

You could easily use them for a complex journey in your own country as long as you meet the criteria on journeys and travel days.

I appreciate this is not the spirit of the 2 days and many will use them to cross borders but I think it is important that new users have the clear guidance that the name is misleading.

In effect the guidance might be better as:

Of your travel days you can travel on up to 2 of them in your home country as if you were outside your country. This is intended to aid any travel plans for using your Global Pass, but is not restricted to nor require any cross border journeys.

In theory, a travel day is considered to be a 24-hour period which lasts from midnight to midnight. Within this period you can take as many trains as your heart desires. However, the real limitation as other travelers have suggested is the out/inbound rule. This rule allows travel within your country of residence only on two days of your total travel days. You can read more about it over here.