It is a special type of integrated circuit socket designed for very high pin counts.
It is present on devices with lower pin counts, as it only requires one pin.
External data memory is not directly addressable except in some high pin count PIC18 devices.
By the time White was conscious to make a pin count, Shamrock kicked out.
These serial memory devices combine the advantage of more speed with reduced pin count as compared to parallel memory.
The pin count was reduced from 44 for the 4096-bit device down to 31 pins and two coaxial signal output connectors.
Of course, this side-band bus increases pin count, power draw, cost, etc., so it's not the most optimal solution.
It is typically rendered as a slash on scoresheets in place of the second pin count for a frame.
Scratch leagues are those in which the actual pin count determines the winner.
Chip carriers are smaller than dual in-line packages and since they use all four edges of the package can have a larger pin count.