What does the searchable field safetyreportid mean in openFDA drug adverse event api

In the openFDA website(https://open.fda.gov/apis/drug/event/searchable-fields), it said safetyreportid is "The 8-digit Safety Report ID number, also known as the case report number or case ID. The first 7 digits (before the hyphen) identify an individual report and the last digit (after the hyphen) is a checksum. This field can be used to identify or find a specific adverse event report." Sorry I was confused with some points of the definition.

1. What does checksum mean? Is that the version number?
2. I saw some case IDs with a hyphen (like ‘4322505-4’) but some don’t (like ‘10003310’), is it just because of the different format?