V3 - What should the strings be in the expanded prop for the labs v-data-table component?

[Carter-Wilson]

Exactly as state in the title - in vuetify v3, we have a v-data-table component in labs, and the docs specify an expanded prop that is an array of strings, but doesn't say anything about what those strings should be. For context I'm working on a v-data-table with custom expanders in each row.