Different meanings, different time periods. The library is not full, which means there are still seats in the library. Seats can be reserved by mobile phone, which means that the overall number of seats is not full. An empty seat in a library refers to the state that no one has reserved it in the time period of one's choice, that is to say, this seat is reserved in other time periods, so this time period belongs to the working state, that is to say, the number of seats in a certain time period is not full.