There are two types of block proposals:
Vote extension enabled block proposal: When vote extensions are enabled, the proposer must include the vote extensions they received from comet within their block proposal - specifically in the first slot of the proposal. We choose to use the proposer to determine the canonical vote extensions for a height because each validator may be maintaining different vote extensions which may lead to in-deterministic results.
Vote extension disabled block proposal: When vote extensions are disabled, the proposer does not need to inject any vote extensions into their block proposal.
When vote extensions are enabled, the proposer will then inject the vote extensions into their block proposal. The reason why this is done is to ensure that the vote extensions can be aggregated and written to state in PreBlock. PreBlock does not have access to the vote extensions used to create the block proposal, so the vote extensions must be injected into the block proposal. Additionally, this allows the network to have a canonical set of vote extensions for a given height.
In the case where the validator does not have valid vote extensions, a new round of voting will be triggered. The validator will then wait for the next round of voting to complete before creating a new block proposal.
The process of constructing the rest of the block is left to the PrepareProposalHandler which is passed into the constructor. This means that process of 'oracle' block building can be compatible with the Block-SDK, which is used to build highly custom blocks.
When vote extensions are enabled, the validator will first verify that the block contains the block proposer's vote extensions. If the block does not contain the block proposer's vote extensions, the block will be rejected. If the block contains the block proposer's vote extensions, the validator will do a basic check to ensure the vote extensions are valid before verifying the rest of the proposal in accordance with the preferences of the ProcessProposalHandler which is passed into the constructor.