There is no such Unique extent !
Extents are logical units for data storage in database. They are combination of 8 data pages i.e. 64 KB.
Extents can be of two types UNIFORM and MIXED.
Uniform Extent - These types of extenct are dedicated to hold data pages for only one type of objects.
Mixed extent - These type of extent can hold different type of data pages like Index, System , Data pages etc.
Not Everything that is faced can be changed,
but nothing can be changed until it is faced.