Single Sleeper - A compartment for 1 person.  It is only possible to reserve a single sleeper with a 1^st class ticket or 1^st class railpass.   

Double Sleeper - A compartment for 2 people with upper and lower beds.  Some trains require the traveler to have a 1^st class ticket or 1^st class railpass.  If you have selected "2^nd Class" and do not see this choice, then select "1^st class".   

Economy Double (T2) Sleeper - A smaller compartment for 2 people with upper and lower beds that is valid with 2^nd class tickets.   

Economy Triple (T3) Sleeper - A smaller compartment for 3 people with upper, middle, and lower beds that is valid with 2^nd class tickets. If a single person or couple book the T3 Sleeper, the passengers will be assigned a place in a cabin with three bunk beds shared with passengers of the same gender.   

Economy Quad (T4) Sleeper - A compartment for 4 people with upper and lower beds that is valid with 2^nd class tickets.   

Couchettes 4 - A smaller compartment for 4 people with upper and lower beds.  Some trains require the traveler to have a 1^st class ticket or 1^st class railpass.  If you have selected "2^nd Class" and do not see this choice, then select "1^st class".  Passengers are usually of both genders unless otherwise stated.   

Couchettes 6 - A smaller compartment for 6 people with upper and lower beds that is valid with 2^nd class tickets.  Passengers are usually of both genders unless otherwise stated.