HP C737990900 Technical Reference Manual page 93

Any mode
page code
3Fh
Allocation Specifies the number of bytes that the host has allocated for returned MODE SENSE data.
Length
0 No data transfer will occur. This is not considered an error.
n The maximum number of bytes which the drive should return in its data-out phase. Either
the entire mode page or allocation length bytes of the page are returned, whichever is
least.
The drive returns that page.
The drive returns all mode pages.
Commands 93