The following information is regarding TuneCore's Publishing Administration service
Your share refers to how much of the copyright you own for a particular composition. For example, if you wrote a composition in its entirety then you own 100%. This means that your share is 100%.
If you have co-writers for a particular composition then you would enter your individual % that is in agreement with your co-writer, and then choose "Add Co-writer" while adding your shares and enter their specific % for that composition. For example, if you and your co-writer agree to a 50/50 share then you would enter your individual 50% share, and choose add co-writer and enter their name and individual share. If you and your co-writer agree to a 75/25 share, then you enter 25% or 75%, for you and your co-writer respectively.
Essentially, the splits always depend on you and what is in agreement with the party you are collaborating with.