Hi
From the manual:
"Checks the parameters of the market that immediately precedes the current market (according to its start time), provided that the current trigger has been applied to that market. That is, the program will look for markets where the trigger has been executed before. If it finds one or more such markets, it will pick the one that starts just before the current market. If there aren't any that fall in this category, the condition will not be satisfied."
so if the specific trigger which has that condition executes only in horse racing markets, it will find the last horse racing market that the specific trigger executed on.
Keep in mind if there was no previously triggered event, the trigger containing that condition will never execute. Use a condition block:
AND
trigger <name> number of runs is equal to 0 overall
or previous triggered events status is settled