lomdus is for questions about the theory of Talmud study or general Talmudic concepts. It is also appropriate for questions about a particular line of Talmudic-style reasoning.
What is a hakira? and Eating outside of a Sukkah - Negative or Positive commandment? are examples of lomdus. In the first, the question asks about a specific type of Talmudic reasoning that is employed in many instances. In the second, the question seeks to employ a Talmudic-style line of reasoning, or at least to identify which one to employ, in order to come to a resolution that can categorize a particular Biblical Commandment.