There is no description for itag 249–251 in Wikipedia.
As note 1 says, “itag is an undocumented parameter used internally by YouTube to differentiate between quality profiles”. There is no public complete list.
Here information from MediaInfo:
249
Format : WebM
Format version : Version 4 / Version 2
File size : 187 KiB
Duration : 49s 621ms
Overall bit rate : 30.9 Kbps
Writing application : google
Writing library : google
Audio
ID : 1
Format : Opus
Codec ID : A_OPUS
Duration : 49s 621ms
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 KHz
Compression mode : Lossy
Language : English
Default : Yes
Forced : No
250
Format : WebM
Format version : Version 4 / Version 2
File size : 229 KiB
Duration : 49s 621ms
Overall bit rate : 37.8 Kbps
Writing application : google
Writing library : google
Audio
ID : 1
Format : Opus
Codec ID : A_OPUS
Duration : 49s 621ms
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 KHz
Compression mode : Lossy
Language : English
Default : Yes
Forced : No
251
Format : WebM
Format version : Version 4 / Version 2
File size : 537 KiB
Duration : 49s 621ms
Overall bit rate : 88.6 Kbps
Writing application : google
Writing library : google
Audio
ID : 1
Format : Opus
Codec ID : A_OPUS
Duration : 49s 621ms
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 KHz
Compression mode : Lossy
Language : English
Default : Yes
Forced : No