What is the difference between ARINC 429 labels 025 and 102 for Selected Altitude?
According to A429, the difference is the type of data coding. Label 025 is BCD. Label 102 is BNR.
Label 025 by being a Binary Coded Decimal codes each character of the altitude value in binary. With a range of 0 to 50000 feet, the first digit uses 3 bits and the remaining use 4 bits each for total of 19 data bits.
Label 102 is a Binary number. The range is 0 to 65535. There are 16 data bits (2^16 = 65536).
Both are transmitted. Receivers will use the version most appropriate for the application. BCD is normally used for display. BNR is normally used in computations.