In this commit:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/usb/storage/uas.c?id=198de51dbc3454d95b015ca0a055b673f85f01bb

There is the following comment:
> 1 tag is reserved for untagged commands

So my question is, what exactly are "untagged commands"?

If we need to reserve (only) 1 tag for them, does that mean "untagged
commands" will never be queued? (I mean, not counting the "tagged
commands"; so in the queue there will always be at most 1 "untagged
command" and a certain number, or none, of tagged commands?)

Also, what determines whether a command is tagged or not? Is it merely
depending on how a program (or maybe the kernel) issue the commands?
Or can a device (or maybe the driver) require specific SCSI command(s)
to always be issued untagged?

In any case, is there a way (e.g. a debug option/level of the sd or
the uas driver, etc.) to show/log whether the issued commands are
tagged or untagged?
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