Ceph » RADOS

My osd tree is like below:

ceph osd tree
ID CLASS WEIGHT TYPE NAME STATUS REWEIGHT PRI-AFF
-7 0.19498 root mytest
-8 0.19498 host test_host
6 hdd 0.19498 osd.6 up 1.00000 1.00000
-1 1.17097 root default
-3 0.58600 host master
0 hdd 0.19499 osd.0 up 1.00000 1.00000
2 hdd 0.19499 osd.2 up 1.00000 1.00000
4 hdd 0.19499 osd.4 up 1.00000 1.00000
-5 0.58498 host node2
1 hdd 0.19499 osd.1 up 1.00000 1.00000
3 hdd 0.19499 osd.3 up 1.00000 1.00000
5 hdd 0.19499 osd.5 up 1.00000 1.00000

But when I created the erasure-code-profile, I specified the device type as ssd, and I can use it to successfully create an erasure crush rule：

ceph osd erasure-code-profile set testprofile k=3 m=2 rush-device-class=ssd
ceph osd crush rule create-erasure testerasure testprofile

But when creating the replicated crush rule, specifying the wrong device type cannot create it successfully.

I would like to know what is the reason for this design