For it is written, way back on page 14, Paragraph F of the Rules of Play The Association will conduct a perpetual "Challenge Board".

Question: What is this challenge board?
Answer: A way for you to challenge a player above your rank, in order to try and gain more rank.
Both players must agree to the challenge. You may play any game the SFDL currently plays in regular season matches.
So get to challenging!
Print out the scorecard here. .

Paragraph F:
The Association will conduct a perpetual “Challenge Board” with the following conditions;
1.   Only challenges between two individual players with rankings who agree to
play in accordance with the table in SECTION IX, Paragraph C: will be used by
the SFDL to calculate new ratings for the two players.
2.   Only challenges played on days other than on days of SFDL approved play
(league or open events) will be approved.
3.   Challenges that comply with the following provisions will be accepted;
(a) All games must be witnessed by a ranked player.
(b) All games must be recorded by an official SFDL scorecard.
(c) The scorecard must be dated and signed by both players and the witness
and mailed on the same day that play occurred.
(d) Not more than sixteen (16) games of 301, 501 or Cricket can be recorded
on a scorecard by the players on any given date.
4.   The Association will provide sponsors with the self-addressed address
scorecards.
5.   The players of the challenge will provide postage.
6.   The Association will provide a new rating for each player for each game that
appears on the postal card in the order they were recorded.
7.   When, and if, a new rating of one of the players is 63 or more points
difference from their rating at the start of their challenge, the calculations
of new ratings will end and any remaining games on the scorecard will be
considered null and void.
8.   No bonus points for highlights will be awarded for the challenge board.