This appears to be where the Physical enclosure disks are defined.

In my system, the following enclosures exist:

• enclosure_0.conf: The primary physical NAS drive enclosure, which can hold up to 6 disks.
• enclosure_1.conf:
• enclosure_3.conf:
• enclosure_33.conf: A generic enclosure ID for all external disks connected to the USB3 ports.

cat /etc/enclosure_33.conf

returns:

[Index]
pd_bitmap = 0x8
pd_sysid_sg9 = 3
pd_sysid_sdj = 3
 
[PhysicalDisk_3]
port_id = 3
att_phy_id = -1
pd_sys_id = sg9
enc_sys_id = root_usb
pd_sys_name = /dev/sdj
pd_ctrl_name = /dev/sg9
pd_bus_name = 18:0:0:0
wwn = D067E516480EF480F618623A
real_wwn = 
type = 2
form = 0
vendor = Kingston
model = DataTraveler 3.0
serial_no = D067E516480EF480F618623A
status = 0
capabilities = 0
sector_size = 512
capacity = 60437492
revision = 
protocol_ver = 0
link_speed = 4
max_link_speed = 0
pd_max_link_speed = 0
read_speed = none
read_iops = none
write_speed = none
write_iops = none
rotation_speed = 0