mi 'gyur rdo rje'i sku - changeless vajrakaya

Well, in some presentations there are actually five parts to buddhahood, sku lnga. In this case there's dharmakaya, sambhogakaya, nirmanakaya, and two new parts.

The first is mi 'gyur rdo rje'i sku - changeless vajrakaya.

Let's go through this term backwards. sku is body, the 'i is a genitive that binds right to what's follows to the left. rdo rje is vajra.

'gyur is to become, and the mi in front negates this, so it does not become, or changeless.

The short form of this is rdo rje'i sku.

For example, vajrakaya is the body in which Jetsun Milarepa arose from his funeral pyre to give one final teaching to his disciples. It is basically the immutable or unchanging nature of the four other Buddha parts.

This is also sometimes translated as diamond body, as vajra is translated as diamond when using this expression, and rdo rje is king of stones, an indestructible stone. Another translation I've seen is body of indestructible reality.

I think to get more in-depth teachings on this topic, it's best to consult with your teachers, especially the Nyingma tradition would have beautiful explanations about this term.

Next, the last part in the sku lnga presentation.