What is meant by "the third" in the hadith "Between every two calls to prayer is a prayer," and do supererogatory prayers have a specific intention for them?

The Prophet's saying, "Between every two adhans there is a prayer," refers to the period between the adhan (call to prayer) and the iqama (second call to prayer, signaling the start of the congregational prayer). The prayer intended here is the absolute supererogatory prayer (nafilah mutlaqah). His statement in the third instance, "for whoever wishes," was said to remove any hardship, meaning that there is no specific number for the absolute supererogatory prayer. Supererogatory prayers are of two types: specified, such as the regular supererogatory prayers (rawatib), which require the intention of prayer and its specification; and absolute, such as the night prayer, for which only the intention of prayer is sufficient.