Interchangeably means​

Interchangeable is something that can be used in the same manner as something else without any important differences.

or in a more simple way

something that can be exchange

For example when two sweaters look almost the same and you could easily wear either one with a particular skirt, this is an example of when the sweaters can be worn interchangeably.

capable of being interchanged

especially : permitting mutual substitution

interchangeable parts

Explanation:

short answer is it means something that can be changed

a.   mass number from atomic number