Some physical quantities do not yet have established notations; their value (and sometimes even dimension, as in the case of the Einstein coefficients), depends on the system of notations.
sˌʌm fˈɪzɪkəl kwˈɔntᵻɾiz duːnˌɑːt jˈɛt hæv ɪstˈæblɪʃt noʊtˈeɪʃənz; ðɛɹ vˈæljuː ænd sˈʌmtaɪmz ˈiːvən dᵻmˈɛnʃən, æz ɪnðə kˈeɪs ʌvðɪ ˈaɪnstaɪn kˌoʊɪfˈɪʃənts, dᵻpˈɛndz ɔnðə sˈɪstəm ʌv noʊtˈeɪʃənz.