In order to work on your store, we need the access to it. We always ask for full store access, however, the choice to which section you'd give us the access to is on you completely. In most of the cases, access to the themes sections should be enough, however, developers may ask you to extend permissions since it highly depends on the task itself.
When we send you a store access request, you will receive it in the form of an email. From there, you need to click on View Request to check which information we will have access to and approve the request. Use the Edit link in the top-right corner of the page when reviewing the request we sent, in order to limit permissions to your store, if you are not comfortable with granting us full store access.
You can adjust the permissions later on through the Account settings to allow us for more access, if this is deemed necessary by our developers.