The activity log is a tool to use to see engagements/interactions with the proposal, both internally and externally. This is strictly activity related per proposal. You can navigate to this by selecting the below icon while in the proposal overview.
Several items are listed Automatically in the activity log:
Automations that have fired off for this proposal
Proposals being Sent and Accepted
Proposals being viewed
Proposals being Updated
Proposals being sent to your CRM (where applicable)
If you would like to manually add a log Select the Plus Icon/Button
If you would like to be reminded about a manually created activity, see Add a Reminder to an Activity Log