Xposed Error

[the bindlestiff]

A reference that describes campaign scripts can be found here. Maybe that will explain it to your satisfaction.

[TomTom]

It is not absolutely required in a Host. (I never bother myself).
Your case here sets the host to run the d_initial string first (the vstr d_initial) which runs the map baserace as the first map and when that "campaign" ends it loads the commands in d1 for the next campaign which runs the campaign with the shortname of baserace. The d1 string also sets itself as the next campaign so that means it will continue to play the campaign baserace ad infinitum.
Where this sort of thing is required is where more than one campaign is to run or where a map has problems being at a certain part of a campaign. In that case you would have something like this;

// CAMPAIGN ROTATION - official campaigns 

// WATCHDOG
set com_watchdog_cmd "vstr d_initial ; say watchdog found no map running - restarted mapcycle" 

// MAP 

set d1 "campaign cmpgn_northafrica ; set nextcampaign vstr d2"
set d2 "campaign cmpgn_centraleurope ; set nextcampaign vstr d1"

set d_initial "set g_gametype 4 ; map oasis ; set nextcampaign vstr d2"
vstr d_initial

// END CAMPAIGN ROTATION

Here d_initial runs oasis then d1. D1 runs cmpgn_centraleurope then d2. D2 runs cmpgn_northafrica then runs d1 and so on and so on....
Note that in this case the watchdog will restart the campaign cycle if something fails to load and stops the campaign (important in a server).

For more on campaigns and cycles the standard page to visit is here.
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The comment in your cfg is key. If running a server as compared to a host you need to have a map running for the console parser to accept the campaign command. Myself I test in a Host situation so I use shortcuts with the campaign command in the shortcut properties (i.e. the command line, faster and better than the menu).