These features refer to the tenth leaf stage. The tenth leaf is the tenth true leaf produced after the cotyledons. On seedlings with alternate or spirally arranged leaves it is just simply the tenth leaf produced. On seedlings with opposite leaves it is one of the leaves in the fifth pair of true leaves produced. On seedlings with whorled leaves it will be one of the leaves in the appropriate whorl; e.g. if there are four leaves in each whorl then the tenth leaf will be one of the leaves in the third whorl.
The petiole is the stalk of the entire leaf, but for the operation of this key, this feature is applied to the leaflet stalk in the case of compound leaves.