You don’t need to “keep the barcode”.

In fact there is no need to pre-activate the pass at all.

In case of a late change of plans it is better not to have the pass active in advance as it cannot be cancelled on the day once active.

The pass is not linked to reservations, as long as you have the required pass active when you travel that is all that is needed. Reservations will still be valid no matter when you activate the pass.

You are advised not to activate the pass until just before boarding the first train of your travel. Please read more about that below. 

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

It may help if you have a simplified guide to how your mobile pass works.

The first thing to repeat is that reservations and your pass are totally independent. If a train does not require a compulsory reservation all you need to travel is your validated pass ( explained next). If your train does need a compulsory pass or supplement (rare but does happen) then to travel you need both evidence of reservation and your validated pass.

Reservations are covered in previous posts,
Your pass is added to the rail planner app, which has 4 tabs, 3 of which you use for actual travelling (The reservation app is not used for travelling with a pass - it simply links you to reservation tools/sites)

Tab 1 is the planner and is simply used, in conjunction with any other planning tool, to identify trains you might want to take and whether or not reservations are needed. It also allows you to transfer planned trains into your trip. Note the planner is an off-line tool and not fully reliable. That is why the advice is to use several tools for research - ask if unsure.

Tab 2 is your Trip. Each pass requires you set up a single trip for the whole use of that pass.
This Trip is simply your diary of trains you intend travelling on. These can be added from the planner or manually and all trains you eventually travel on must be listed in your trip. These can be added to and removed at any time and, like paper diary entries, are simply a wish or action list. They are not bookings or train tickets at this stage,
So to the key part - Your pass. Once activated, ideally just before you board your first train - see earlier posts - you can transfer the train details for your first train from your pass by moving the slider. This creates a travel day in your pass, a QR code is generated and the train details shown under the QR code. Thls is now a valid ticket for that train. If you board any more trains that day you follow the same process of transferring the new train to your pass so it adds to the list displayed below the QR code.

On subsequent days transferring your first train will again create a travel day and a new QR code and listing.

I believe you need access to the internet to create the travel day (others may correct that) but once the travel day is created everything else can be done off-line. The app does need access to the internet every 2 days to update, but it is not needed during actual travel.

I hope that helps.