8 hats

Hello,
I would like to ask question with regard to IRR. In the answer, it is stated that IRR has no effect when selecting project with the same scale of investment. IRR is important when:

1) Project has pre-specified co. rate
2) Project with different scale of investment is being compared.

So my questions are:

1) What does it mean by company pre-specified rate?
2) What does it means by different scale of investment?

From my understanding, IRR is not beneficial for mutually exclusive project like in this case due to: 1) different in initial investment & cash flow.
Is my understanding correct?

Thank You