If you don't know what this is, I suggest you start by reading the Kernel-HOWTO available from the MetaLab website Kernel-HOWTO or from the TSX-11 ftpsite Kernel-HOWTO, or from their respective mirrors. See also www.linux-howto.com.
The functionality of a card is split into many different logical devices for convenience. The idea is that each logical device is independent and has it's own driver.
This way, the user can decide to use as much or as little of the card functionality as required, and also allows common drivers for many different boards (the drivers being written for each logical device function).
So for example a soundcard will typically have at least four different logical devices:
The logical devices are numbered starting from 0, incrementing by 1. The first entry in the resource data applies to logical device 0, the next to logical device 1, and so on.
Each logical device has its own set of resources ( IO addresses, INTerrupts, DMA channels etc) to program, and isapnp uses the LD command to select the logical device to be configured.