What can be included in a contextual window besides user messages? Why can a new empty chat be filled up to 15%?

In the context window, in addition to user messages, there can be:

• Assistant (bot) messages to maintain the dialogue and consider previous responses.
• System instructions or prompts that define the model's behavior.
• Metadata, such as information about the user, settings, or dialogue goals.
• The history of previous sessions or messages to preserve context.

A new empty chat can be filled up to 15% because system prompts, instructions, welcome message templates, or basic settings are already loaded into the context. This is necessary for the correct operation of the model and the formation of relevant responses, even if the user has not yet entered their messages.