It's slot pairs.
This is from the IAG press release
IAG has offered the following commitments to the EC as part of the regulatory process:
Seven daily slot pairs to be used between Heathrow and either Edinburgh and/or Aberdeen.
Five daily slot pairs to be used between Heathrow and the following destinations – Nice, Cairo, Riyadh, Moscow, Edinburgh and/or Aberdeen.
Two Heathrow daily slot pairs will be leased to Transaero for use on flights to Moscow.
Other airlines can apply for seats on the integrated BA/bmi short and midhaul network for their transfer passengers, on normal commercial terms.