Calamansi is a small citrus fruit with tart juice and aromatic peel. It is widely used in Filipino dipping sauces, marinades and drinks, including combinations with soy sauce. The fruit may be used while green or after its peel turns orange.
Lime or lemon can supply acidity when calamansi is unavailable, but neither reproduces its aroma exactly. Bottled juice and sweetened calamansi drinks are different products; check ingredients before substituting.
Reference: Further reading.