Understanding projection layer for BLSTM

In many research papers there are 'projection layers' related to BLSTM layers. For example, from here:

"we trained an 8-layer BLSTM encoder including 320 cells in each layer and direction, and the linear projection layer with 320 units followed by each BLSTM layer"

I can't understand what this means and how it works.

Any help on this topic would be appreciated. Thanks in advance!

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It is called projection because you "project" a representation of dimensionality $$M$$ into a representation space of dimensionality $$N$$.