The following information is regarding TuneCore's Publishing Administration service
Your split 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 split 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. For example, if you and your co-writer agree to a 50/50 split then you would enter your individual 50% split. If you and your co-writer agree to a 75/25 split, then you enter 25% or 75%, whichever was granted to you per your agreement with your co-writer.
Essentially, the splits always depend on you and what is in agreement with the party you are collaborating with.