Japanese term (edited): ���b�Z�[�W�����폜

You may be familiar with disk formatting - there is logical formatting and physical formatting (also sometimes called low-level formatting)
I think this means that the message should be deleted physically, or at the low level. What I mean by this is:

Usually when a file or object is deleted from a computer system, to allow high performance, the bytes occupied by the object on the disk are not actually erased. Only the record pointing to the disk location is removed from whatever the computer system is using to keep track of those things. (For a file system, the pointer to the file is removed, but the data remains on the disk).

For disk formatting as well, a logical format only rewrites information about how the disk is organized, but does not erase any of the bytes previously stored on the disk. A computer technician can go back and get data off the disk if needed.
This is not possible with a physical format, where each bit of data on the disk is overwritten with zeroes or random data. That is usually a time consuming process.

So, if you understand that, the same principle probably applies here. The pointer to the location on the disk or in a database for a message is not only deleted, but the location that the message occupied is also erased, which prevents future attempts to recover it.

The phrase seems to be discussing exactly when physical (and non-reversible) deletion of a message should occur. It then mentions that the message deletion method is not defined but if there is another way of doing it, to note that.

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Note added at 3 hrs 1 min (2005-02-04 02:06:00 GMT)
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Noting KathyT\'s suggestion, but also this is sometimes called a message purge.