The Insights API can return several metrics which are estimated or in-development. In some cases a metric may be both estimated and in-development.
Estimated - Provide directional insights for outcomes that are hard to precisely quantify. They may evolve as we gather more data. See Ads Help Center, Estimated metrics.
In Development - Still being tested and may change as we improve our methodologies. We encourage you to use it for directional guidance, but please use caution when using it for historical comparisons or strategic planning. See Ads Help Center, In development metrics.
For more information, see Insights API, Estimated and Deprecated Metrics