Most radios today have the ability to scan a range of frequencies. Referred to often as Program Scan or Programmable scan or PMS, these all work basically the same way. Given two frequencies, the radio scans between the two. These two can be small segments of the band or can cover a wide range over a greater difference depending on where the activity is that you're looking for.
In the RT Systems programmers, you generally will find these special memories labeled as Limit Memories.... they are the scanning limits, ergo the name. Sometimes they are complete memory channels and can be used as additional memories if you have no plans to use this scanning. That is valid use for these memories... in some radios. Often, the entry will include all the details of a memory channel even is the radio is not using all those details for scanning.
In the Kenwood TS-590S and TS-590SG, the limit memories are pairs of frequencies that define the upper and lower limits of the scanning range. The P0 - P9 that you see are those ranges. Enter the lower and upper frequencies for this range. Ranges can overlap. The radio will step from the top of one range to the beginning of the next when more than one is active.
Once you enter the frequencies and send the file to the radio, tell the radio which of the groups are to be scanned.
When scanning, the radio will display the group number on the left P1, P2, etc and may display Scan P1 on the right. The Scan P1 is there to indicate the scan speed where P1 is the fastest and P9 is the slowest.