A brain-computer interface records from or stimulates a brain to exchange information with an external device. A neural biocomputer uses living neural tissue as part of the computational hardware. The distinction is purpose and system boundary.
Biocomputer
Brain-computer interface
Neural tissue
Usually cultured in vitro
Usually a living person's or animal's nervous system
Main objective
Use neural dynamics for computation or research
Communicate with, restore or augment nervous-system function
System role of neurons
Computational substrate
Source or destination of neural information
Shared engineering
Both fields use electrodes, stimulation, decoding, signal processing and closed-loop control. Advances in high-density electrode arrays and flexible interfaces can therefore cross between them.
Why the language gets blurred
An in-vitro neural culture coupled to a robot can look like a brain-machine interface experiment, while the same platform can be described as biological computing when the neural network is being evaluated as an information-processing substrate. The research question determines the more useful label.