CGU exam kit

I just want to know what they mean by no asset should be impaired below their recoverable amount.

The following is to be used for the question 23 and 24.
A division of a company has the following balances in its financial statements:
Goodwill $700,000
Plant $950,000
Building $2,300,000
Intangibles $800,000
other assets $430,000
Following a period losses, the recoverable amount of the division is deemed to be $4 million.
A recent valuation of the building showed that the building has a market value of $2.5m. The other net assets are at their recoverable amount. The company uses cost model for valuing building, plant and equipment.

23. To the nearest thousands, what is the balance on the building following the impairment review?
Kaplan responses:

Answer $2,300,000 , reason, $2.5million can not be chosen as the company uses the cost model. Also, in CGU, *no asset should be impaired below its recoverable amount*. The valuation of $2.5m is the property is not impaired and should remain $2.3million.

Qstn, was the $2,5m valuation the recoverable amount? as against the entire division recoverable amt of $4million? Total impairment is $1.18 ($5.18 – $4). Obviously it was not impaired because the recoverable amount was higher than current carrying value of $2.3m and also because of the cost model used but what if we use revaluation model ?

24. To the nearest thousands, what is the balance of plant following impairment review?

total impairment is 1.18
dr impairment exp 1.18
cr CGU 1.18
only plant and intangible asset were allocated impairment and also goodwill
Goodwill writing off first nil remaining $480,000(1,180,000- 700,000)
total c.a of plant and intangible asset was $1.75(950,000 + 800,000)

plant impaired =$261,000
c.a is $689,000
Fine this is clear but i just to know why building and other asset was not included as this voice down to my curiosity of above* no asset should be impaired below is recoverable amount. Meaning no impairment should be allocated to these building and other asset*.

Ignore every calculation as i do understand, but throw more simple explanation the above rule.

Thanks