This paper deals with non-restrictive infinitival relative clauses (NIRCs) in English (e.g. An independent review, to be funded by the health authority, has been commissioned). The purpose of this paper is twofold: (i) to give an accurate description of the semantic properties of the NIRC on the basis of the British National Corpus, and (ii) to elucidate restrictions on the use of this construction. My corpus-based approach will clarify four types of shades of meaning expressed by this construction are equal to those expressed by IS TO construction (i.e. plan, necessity or appropriateness, future in the past, and possibility). I will stress that NIRCs are used only when they have one notional category (i.e. notional subject/object), which is highly likely to be a notional subject of the infinitive, and denote one of the shades of meaning rather than causality.