SNDCURVE is a raw sound lookup table. Heretic and Hexen make use of it, though ZDoom does allow you to use them in other games as well. Such tables are used for any sound that specifies that it uses $rolloff custom in SNDINFO.
Typically this is a square table of bytes; for example Heretic uses a 40x40 table (1600 bytes) and Hexen uses a 45x45 table (2025 bytes). ZDoom, however does not require the lump to be square: the table is actually treated as a one-dimensional array.
The rolloff value used is the one at an index determined by the position of the sound origin's actual distance between the maximum and minimum distances: the smaller the distance, the lesser the index in the table. A value of 127 means no attenuation; greater means a greater volume and lesser means a lesser volume. Typically, the values scale between 127 at index 0 in the table, and 1 at the very last index.