There are three types of passes:

• continuous global passes
• flexi global passes
• One-country passes

Continuous passes allow for unlimited travel in between 7 and 90 consecutive days. All days you can travel unlimitedly in Europe.

Flexi passes allow you to chose a number of days in a period (either 1 month or two months, not necessarily natural months) which you can travel. For example, the 10 days in 2 months pass allows you to chose 10 days in any two-month period in which you travel.

For example: You travel on day 1, then spend two days (4 nights) in a city and on day 4 you take a train again. The two days you've spent in a city you don't need to use a travel day for, so you would only have used 2 travel days. 

Both these Global passes allow for unlimited travel in Europe, except in your country of residency (if you're a European resident). Note you'll need seat reservations which can add quite a bit to the cost.

The third type doesn't allow travelling all over Europe: these are one-country passes, which only allow travelling within one country (with some exceptions). 

None of the passes allow spreading it over a year. You can maximally spread the travel days over 2 months if buying a 10+ day flexi pass, all smaller flexi passes only allow spreading it over one month. You can buy a 3 month continuous pass too, but that is three continuous months you can travel (about 90 consecutive days).

There is no such thing as "fixed tickets" with Interrail/Eurail. You have flexible passes X amount of travel days to be used in 1 or 2 months. Activation possible from purchase until 11 months after it. Once activated, you can use it until your travel days are all used or the 1 or 2 months subsided, if you didn't use up all your travel days. 

Continous passes are valid for 15 consecutive days or 1 month, or 2 or 3 months after activation. You cannot spread these over the year. Once activated, it's valid for max 3 months, depending on the pas you bought. 

You cannot spread your passes over a year. If you plan 2 trips e.g. One in March and One in December you'll need to buy two passes.

You can take whichever train, as long as the train company is participating in the Interrail /Eurail scheme. ( about 95% of all train companies in Europe). But with a flexible four days in a month pass, you won't be able to see that much as you would with a continious 3 months pass of course :) 

IF you apparently want to save some money: IF you can plan it to put those 30 days as 3x10 in 2 monthes-spread over the yr, then you could thus handle it with these 3 passes. Up to you how you spread them then, but if its for a regular return every 3-4 weeks this will not be possible that way. IF that would even save money is also wholly dependent on what sector and for which trains.

BUT tis will also lead to problems with the sector in home country, DE for you: as each pass only allows 2 days in there- for going out and back in. Thus it is not possible to do 5 returns on a 10-day pass without paying extra. Again this wholle depends on what trip what is günstig oder nicht.

Have you won a pass from the DiscoverEU?

You should  find the answers to questions here:

https://youth.europa.eu/discovereu/faq_en#deu-faq-C

 @BrendanDB @Schelte  @mcadv  The possibility to use Interrail given by DiscoverEU is a bit different than ordinary Interrail passes.

The flex ticket from DiscoverEU is 7 travel days within a month. You can travel in your country of residence on a maximum of two of those travel days.