ticket with no seat assignment

Do you mean a reservation? Since your pass is your ticket.

You can take any free seat where no reservation is indicated. There should normally be a small display to indicate where a seat has been reserved.

Thank you that is good to know. Yes our reservation is not assigned to any seat and I don’t want to just seat ourselves in someone’s spot.  

It is still strange though. There should normally be a coach and a seat number.

For which exact train is this? (origin, destination, date, departure time)

You can take ANY train-and thus adjucs the ´schedule´ very much to minimise these idiot stays in cold+nasty stations. You CAN fo just 4€ REServe a secured seat-IN DE even with 1 connection too, via bahn.de ´seat only´ (actually Ie just done that for a next trip on my current pass for a night train). The only thing you then get is a mail with car+seat nr.

f.e. in this case stay on train to FRA airport and take the same train toward L. OIR stay onbtrain till f.e. Köln and chenage=will mean later arr in the biggest head-on station (or giant shopping centre) that is L since a few yrs

In Switzerland you normally don't need any reservations. It's only a waist of money. There are some scenic routes that requires reservations but all normal trains you just enter without reservation. 

In Germany you can travel without reservation as well but if I understand the information correctly the reservation in first class is free at DB. https://www.bahn.com/en/offers/Seat-reservation-and-other

It's only free in first class if you buy a normal ticket at the same time… so if you have a pass, you need to pay for a reservation.

I assume that you have a "normal" pass and not a flexi pass, as this trip will take two travel days. 

If you travel during the day the trip from Interlaken to Leipzig take about eight and a half hour. You can leave Interlaken at 14.00 and be in Leipzig at 22.24. 

I use the SBB (Swiss Railway) planner. https://www.sbb.ch/en/home.html

We do have a 1st class flex pass and as I am trying to figure out the easiest route I do see that even when it states reservation needed it’s only for 2nd class not 1st. We are doing all our travel thru the nights so we can see as much as possible over 3 weeks. Our goal is 7 countries! We are making longer stops as we have families in Poland and Germany. This trip was 3 years in a making due to COVID. It appears as all countries that we are traveling to are lifting most of the requirements. Italy asked for a entry form. We are fully vaccinated. 

Hi, 

I think you can do the same route without standing on the platform at night.

DB has some good IC/ICE-connections at night. You can check this map:
https://assets.static-bahn.de/dam/jcr:b74478d1-e001-410b-8c4f-8214ef79b2f4/20211130_ICE_IC_EC_Nachtnetz%20FP22-final.pdf

19:00 Interlaken - 21:01 Basel (IC)
21:13 Basel - 08:00 Berlin (IC)
08:30 Berlin - 09:42 Leipzig (ICE)

All the information is extremely helpful so thank you very much. We are doing a tour so our time when we can travel is more restricted. I am hoping that we can catch the train at 19:00 but it will be close. 

If you can’t make 19:00, there’s also an option at 22:00, on which you can stay until overnight until 5.24. 

Interlaken 20:00 - Basel 22:01 (IC)

Basel 23:13 - Bonn 05:24 (IC)

Bonn 06:23 - Hannover 09:28 (ICE)

Hannover 09:36 - Leipzig 12:16 (IC)

From Bonn, you can also take an ICE Sprinter (reservation required). Then you can be in Leipzig an hour earlier, at 11:10. 
 

—- 

update: no reservations required. 
 

Mandatory reservations in ICE Sprinter trains were scrapped a few years ago.

Thank you everyone this is so helpful