Assuming a 3.5" or 5.25" diskette placed on a surface with its label side facing upwards, which side of the magnetic disc within does head #0 of the drive reside on?
According to ECMA-125, a standard specifying high-density 90 mm floppies (so-called “3½ inch”), §4.14:
Side 0 is the side engaged by the spindle. Side 1 is the opposite side.
The spindle engages the disk from the bottom, i.e. the opposite side of the label.
For convenience of description the two sides are defined as Side 0 and Side 1; they are shown in Figs. 1-4 and Fig. 8.
However, the figures clearly show that side 0 is the one which puts the index hole window on the left:
So it is again unambiguously identified. Whether it should be called the “top” or the “bottom” is, however, much more debatable: 5¼ inch disks never had as strong a convention about it (or the placement of the label) as 90 mm floppies.
Side 0 is on the 'bottom' side of the drive, or more exactly it's on the non-moving side when closing/opening that drive. Which is, at least with all early drives also the side the drive motor is acting on. That side is traditionally mounted bottom to improve adjustment but it's also a more natural handling to close downward.
This is historically due to the very first 8" drive being single side. Having the head mounted on the non-moving side simplifies mechanics. For later two-sided drives it wouldn't have mattered, as one head must be on the closing side, but numbering sequence had to be kept for compatibility when operating single-sided disks on dual-sided drives.