Each pin on the component has two nodes, a die-side node and a board-side node (See the probe_die_side parameter discussion in Edit IBIS Component Properties). You can probe both nodes, but you can connect only to the board-side node. Between the die side and the board side, the package introduces RLC parasitics. Accurate modeling of these parasitics is required for signal and power integrity analyses.
By default, the RLC data is specified in the [Package] keyword in the IBIS file. The [Package] entry defines a range of values for the default RLC of the component pins. The IBIS file [Pin] keyword can contain values for the RLC parasitics on a pin-by-pin basis. If the [Pin] entry for the component includes individual RLC values for a given pin, they override the defaults in [Package], for that pin. The pkg_selector property defaults to Pin. With this setting, Nexxim searches the IBIS component definition for a [Pin] keyword containing RLC data for the selected pin, and applies that data if the [Pin] entry is found. If no [Pin] entry is found, Nexxim uses the default [Package] data. Also select Package to have Nexxim skip the search for a [Pin] entry.
If the IBIS file contains a [Package Model] keyword and you select Use [Package Model] at import time, the model RLC values override the [Pin] and [Package] values for the imported pins that are included in the Package Model Definition. The pkg_selector property is not displayed for the imported pins that have a [Package Model] definition. Any imported pins that are NOT included in the Package Model definition are handled as though their pkg_selector is set to [Pin]).