It's a simulation of the the SADL message packet train. The SPI first comes down as a linked coordinate, then is identified as a SPI, IIRC.
Eddie nails the Mark Z answer.
Has anyone noticed the OTHER mysterious TAD symbol? The white hollow diamond? It seems to be related to datalink SPIs from other aircraft. Usually it's only up briefly before being replaced by the usual two-tier wedding cake "mini SPI."