It is not modifiable on the context plans Accounting and financial nor on the IAS/IFRS plan.
When the Depreciation basis origin of the plan has the value IAS/IFRS, the exchange rate type is filled by default by the type defined at the level of the IAS/IFRS valuation on the Principal tab ; otherwise, it is filled in with the type defined at the level of the context definition which the plan belongs to. It remains modifiable.