Chat. New channel for a topic or a project; New direct message for one person. Private channels are visible only to the people in them.
The point of it being here rather than in another app is that the invoice, the project and the conversation about them are one click apart. A question about a job asked next to the job does not need to be explained first.
What it is not
Not for customers. Customers reach you through the virtual assistant's inbox — calls, texts and WhatsApp — which is a different place on purpose.