As long you do Liverpool - London - Paris in one day it´s one outbound day :)
The Outbound journey means you have one travelday to leave your country with as many trains you need Same applies for the inbound journey :)

Thanks seewulf. I had the same question. I asked in the chat bot and it said “You can take as many trains as you need during your outbound/inbound trips, as long as they end within the same travel day in which they start and are either directed towards the border or your hometown. So you cannot use them to travel around in your country.”
But do you know if that has to be one “journey”, or can I put in each section separately? When I put in Bicester (my home station) to Paris, it shows a different combination of trains from what I wanted to use. 

You can put different sections in My Trip for the inbound/outbound journeys. The planner in the app is not very sophisticated, so sometimes you need to split journeys up and that is no problem.

Thanks Seewulf that’s the certainty I needed! Legend! 

I think the outbound/inbound travel day means I have one day to start the final trip of my journey out of/into my home country...so I can be on a train leaving/getting home after the outbound/inbound travel day

@marcas 

If you need to travel more than the outbound and inbound travelday in you home country you need to pay for those trips.

so if my home country is the UK & I catch a train from London to Paris leaving at 11.45pm on my outbound day, I need to pay for the portion of the ticket from 12.00am to the time the train actually leaves the UK?...

no as you boarded the train at 11.45 pm :)

Nighttrain rules.
If you board a train and doesnt change after midnight it will be counted as one travelday (the day of depature)

@marcas The travelday starts at midnight CET which is 23.00 GMT. This you have to be aware of when travelling in the UK.

And that the Caledonian Sleeper just doesn’t play by night train rules at all! 

They've recently changed that:

See here:

https://www.sleeper.scot/tickets/

There are no overnight trains from Britain to anywhere as Eurostar does not run during the night so it is not possible to stretch an outbound journey into a second day.

The best you could do is travel to ports such as Harwich, Newhaven, Portsmouth, Rosslare, Holyhead, etc. to get an overnight ferry.

In the other direction it is possible to use the sleeper services to stretch your inbound journey into the next day following a daytime Eurostar.