This is the 2996th most frequent Russian word.
дополнительную
Additional.
The word 'дополнительную' is used here as an adjective to describe an extra battery purchased to complement the main one.
Я купил дополнительную батарею для телефона.
I bought an additional battery for the phone.
In this sentence, 'дополнительную' describes a room that was added to fulfill the specific needs of the project.
Для проекта нам выделили дополнительную комнату.
An additional room was allocated to us for the project.
Here, 'дополнительную' modifies 'задачу', highlighting that the assignment is an extra task beyond the regular ones given.