Alignment....
When RSNetworks Auto-Maps the devices into the Input and Output tables, it just "fills" them from the bottom to top.
Since these tables are 32-bit wide, and DeviceNet data is 8-bit "byte-sized", often multi-byte data from a device can get split across two DINTs in the I/O mapping tables.
The alignment options just tell RSNetworks to start the next device at a new "boundary" to make the mapping neater. If I remember correctly, this new boundary can be the start of the next 16-bit, or, more commonly used, at the start of the next 32-bit word.
I don't like this auto-mapping, as it always makes the I/O interface tables look ugly.
What I do is map each device into the tables manually, at 2 x node address. This allows for up to 8-bytes of data for each node. If a node uses more than 8 bytes (not common), I then skip the next node address to allow the data to "overflow" into the next 8 bytes.
It fits in well with the I/O tables' sizes too, since you don't allocate node 63 to a device (default address), and common practice is to reserve node 62 for your programming/commissioning device (eg. 1770-KFD). Highest node address would then be 61, which would map to DINTs 122 and 123.
Hope this all makes some sort of sense.