The final version of the documentation for Maya is still under construction. This temporary version is based on the Blender documentation, though its contents is relevant to Maya as well (both variants use the analogical UI).

Pixel Margin

Watch the following part of the tutorial for an introduction to the functionality:

The Pixel Margin functionality allows the margin of the resulting UV map to be determined in pixels of the active texture. In order to use this option, enable the check box located in the Pixel Margin subpanel header:

With the Pixel Margin functionality enabled, the Margin option from the Packing Options subpanel will be ignored and the packer will determine the correct pixel margin set by the user in the Pixel Margin parameter.

Pixel perfect alignment

When the Pixel Perfect Alignment option is enabled, the packer will round bounding boxes of islands being packed to the nearest pixel coordinates during packing.

Check the example:

There are some considerations of this option which need to be taken into account though:

the Pixel Perfect Alignment functionality is currently expected to work properly with islands of rectangular shape only. Bounding boxes of irregular-shaped islands may not be pixel aligned as described below but you can still pixel align their vertices using the Vertex Align Mode option (read below),
when the option is used with Fixed Scale off, the packer may apply slight non-uniform scaling to particular islands in order to align both dimensions of the island bounding box to the pixel coordinates. ‘Non-uniform’ in the sense that the scaling factor applied to the X dimension may be slightly different than the factor applied to the Y dimension. At the same time, the packer will apply scaling of the lowest possible non-uniformity required to align the given bounding box to the pixel coordinates,
when the option is used with Fixed Scale on, the packer is not allowed to scale islands during packing. That is why it can guarantee pixel perfect alignment of left-bottom bounding box corners only. If you want all bounding box corners to be pixel aligned, you need to make sure the islands are already aligned before running a packing operation,
the Pixel Perfect Alignment option determines the pixel coordinates using the current value of the Texture Size option. Make sure you set the option to the dimension of the target texture before the packing operation in order to get consistent results.

Keep in mind the Pixel Perfect Alignment option aligns island bounding box corners only - island vertices are not pixel aligned by default. If you want some of vertices to be pixel aligned after packing is done, set a proper value for the Vertex Align Mode option. The option determines which vertices will be automatically pixel aligned after packing is done. The available choices are:

None - none of the vertices will be pixel aligned (the default value).
Bounding Box Corners - only vertices located in corners of island bounding boxes will be pixel aligned.
Bounding Box - only vertices located on island bounding box edges will be pixel aligned.
Border Edges - only vertices belonging to island border edges will be pixel aligned. Note it also includes border vertices on inner island holes.
All - all vertices will be pixel aligned.

Be careful when setting this option as pixel aligning too many vertices may introduce degenerated faces in the UV map.