Just received that in email. The "certain securities" mentioned are:

Due to ongoing market volatility, the following securities are currently set to position-closing only:

AAL

AMC

BB

BBBY

CTRM

EXPR

GME

KOSS

NAKD

NOK

SNDL

TR

TRVG

This means you can sell and close your positions, but you can't open new positions.

Any open orders (such as market orders and limit orders) for these securities were canceled, since they would have resulted in a new open position. If you hold any of the listed securities, you can only close your positions at this time.

For more background about how to be an informed investor, read our blog post from this morning.

Just received that in email