The use of Pan Pan indicates that the pilot considers this is be an Urgent situation. More information on urgency messages and also the various message categories is found below...

CATEGORIES OF MESSAGES
The messages transmitted by either the aircraft or the aeronautical stations can be classified into categories. These categories are defined by ICAO Annex 10, Volume II.

The categories of messages handled by the aeronautical mobile service and the order of priority in the establishment of communications and the transmission of messages shall be in accordance with the following table (Message category and order of priority):

1) DISTRESS calls, distress messages and distress traffic
2) URGENCY messages, including messages preceded by the medical transports signal
3) Communications relating to DIRECTION FINDING
4) FLIGHT SAFETY messages
5) METEOROLOGICAL messages
6) FLIGHT REGULARITY messages

ICAO Annex 10, Volume II
5.3.3.1 Action by the aircraft reporting an urgency condition

5.3.3.1.1 In addition to being preceded by the radiotelephony urgency signal PAN PAN, preferably spoken three times and each word of the group pronounced as the French word “panne”, the urgency message to be sent by an aircraft reporting an urgency condition shall:

a) be on the air-ground frequency in use at the time;
b) consist of as many as required of the following elements spoken distinctly and, if possible, in the following order:

1) the name of the station addressed;
2) the identification of the aircraft;
3) the nature of the urgency condition;
4) the intention of the person in command;
5) present position, level (i.e. flight level, altitude, etc., as appropriate) and heading;
6) any other useful information.