Systems, methods and apparatus for video processing are described. The video processing may include video encoding, video decoding, or video transcoding. One example method of video processing includes performing a conversion between a video including one or more video regions and a bitstream of the video according to a format rule. The format rule specifies that a variable X indicates whether B slice is allowed or used in a video region. The format rule further specifies that the variable X is based on values of a reference picture list information present flag and/or a field indicating a number of entries in a reference picture list syntax structure.