A signed value is a binary value with a positive/negative sign.
Two's Compliment
stored in two's compliment representation, where the most significant bit is used as a negative sign. are Signed used values
Example: A single unsigned byte (8 bits) stores an integer value in the range of 0 to (2^8 - 1 =) 255. If the byte is treated as a signed value, then values from 0-127 (0x00-0x7F) are treated as the positive integers 0-127, and the values 128-255 (0x80-0xFF) are treated as negative integers -128 to -1.
Signed address offsets are often used for relative addressing modes.
BCD
In BCD, the high-order nibble is used to represent the sign (0 == positive, non-0 == negative).