This is a non-SI unit, and therefore its use is discouraged. It is listed in Table 8 of the SI brochure ("Other non-SI units currently accepted for use with the International System"). BIPM explains "Table 8 lists some other non-SI units which are currently accepted for use with the SI to satisfy the needs of commercial, legal and specialized scientific interests. These units should be defined in relation to the SI in every document in which they are used. Their use is not encouraged.".
Nanometres or picometres can easily be used instead. However, despite its official deprecation, some scientists continue to use it. Some claim it is a more convenient unit, corresponding closer to the size of the items being discussed, e.g. atoms, grains of interstellar dust, optical wavelengths, etc.
See also: Conversion of units.\n