What is filling a cake is it a filling between two layes or is it something you put in a cake mix Like lemon craem cake or lemon chiffon cake.
The filling is something that is placed between the layers.
Some popular fillings are fruit based. Butter cream can also be used as a filling. Other fillings may be mousse, custards, curds etc.
So anything you may spread between the layers is called a filling.
Anything I put between 2 layers of cake I consider filling, even if it's just butter cream.