The number 9 at the start of a phone number in Spain indicates it is a fixed landline. The next digits after 9 indicate the province or geographical region to which the number belongs. For example, 91 corresponds to Madrid, 93 to Barcelona, 95 to several provinces in Andalucía including Sevilla (954) and Málaga (951), and other numbers indicate provinces like Valencia (96x), Asturias (98x), Galicia (98x), and so forth. Each province has one or more specific prefixes following the initial 9 that geographically localize the phone number. This system helps identify from which part of Spain the landline number originates.

So, the digit 9 belongs to fixed phone numbers in Spain, and the exact province or region can be determined by the digits following 9 in the phone number.