Documentation says:

The custom field context allows your custom field to be configured differently for numerous different combinations of issue types and projects. For example, your custom field could have different default values for different projects (and/or issue types).

I want to emphasize the (and/or issue types). This seems to imply that you can set multiple default values within the same project for different issue types.

After some testing, I found that once you have selected and configured a specific project/issue type combination, you cannot select that same project again for editing. This means that you cannot set the 'different values' for a different set of issue types within that same project.

Example:

• I have the project 'ape planet' and want to create a custom select field with values A, B, C for the 'Bug' issue type and values 1, 2, 3 for the 'Improvement' issue type of this project.
• As shown below I did the following:
  • Created the new custom field select type, selecting 'Bug' and 'ape planet' context.
  • I then clicked 'Add new context' to now configure the 'Improvement' issue type, but I find that 'ape planet' project can no longer be selected.