The rule in question relates to tyre usage for the three-part Saturday shootout session that determines the grid for the sprint later in the day.
The intent of the rule is that teams have to use new medium tyres in SQ1 [Sprint Shootout Q1] and again in SQ2, and then use new softs for the final SQ3 session.
However, the rule as written doesn’t actually oblige teams to save a new set of softs for Saturday.
It reads: “In the period SQ3 of the sprint shootout, up to one set of dry-weather tyres may be used, and this must only be a new set of the soft specification.”
It means that a team that doesn’t anticipate it making Q3 or SQ3 in normal circumstances can gamble on not needing a new set of softs for Saturday.
Instead they can deploy all their soft tyres in Friday qualifying for Sunday’s Grand Prix.
Source: Autosport