Switch Tenor

Key Switch Tenor in Fx Vol Spec refers to an optional time period - expressed as an object of type Step - that defines when the long term definitions specified in LT ATM Def and LT Delta Def are valid.
More precisely, the period here is added to the trade date to produce a switch date Tˢʷⁱᵗᶜʰ, which acts in the same manner as if it had been entered through the key Switch Date

If the entry here is left blank, the Tˢʷⁱᵗᶜʰ is defined by a potential entry in Switch Date
If the latter is missing, the Tˢʷⁱᵗᶜʰ is set to infinity, which means the LT ATM Def and LT Delta Def entries are ignored.
If the entry here is provided, the Switch Date entry must be left blank.