I suppose with GB you mean "global”. (I first though Great Britain...)

Optional means optional. It's up to you whether you want to make a seat reservation. Some operators will show how busy they think the train will be, e.g. DB (bahn.de) or ÖBB (tickets.oebb.at).

Required means you have to have a reservation to be able to use that train.

For the best ways to book reservations, check this page:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Please note that you don't need to activate your pass to be able to make reservations. Moreover, it's not a good idea to activate a pass in advance. Plans can change, eve et the last minute e.g. because you wake up ill on the planned start date. You can only cancel activation until 23:59 CET before validity starts and after that it's too late and you'll loose travel days.

Bingo you are right, sorry for my short form GB, I mean "global”, thanks.

I believe the date should not be changed, at most it will be change the hour only, because I came by plane.

BTW,  I found out the trip or cart I made from mobile app, that would not shown on my website (desktop), even made by website, the mobile app also not found. I just worry that after I make any reservation on website or mobile, on the otherside can be find my booking.

The journeys you add to the app won't be visible on the website. This is normal, since app and website are (almost) completely separate. Also, reservations and pass are completely separate. This is normal because there are a lot of places where you can book reservations (usually cheaper than via Eurail) and Eurail can't know about anything you booked elsewhere.

I already made a first 2 day planning through online website order. But now I’m very confuse that my website order did not shown on my mobile app. Even I got the email attached the train pass by pdf files.

1.  Is it also seperate to app and web?
2.  My mobile app can / cannot shown my e-ticket 
3.  In my understanding, Mobile app only can show ticket (qrcode) that are free train ride.

1. Like I said, Rail planner app and Eurail website are separate.
2. Please check this page how to use the mobile pass and to show the ticket:
   https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started
   If that doesn't work for you, then please send a screenshot of where you get stuck. Also note that the timetables valid from 15/12 are not yet available in the rail planner app.