15 characters or 18 characters?
Salesforce record IDs can appear in both forms. ReadyRows checks the basic ID shape before an Update or Upsert file reaches the importer.
Both lengths exist
Salesforce uses 15-character case-sensitive record IDs and 18-character case-safe IDs. For integrations and spreadsheet workflows, the 18-character form is generally easier because its extra characters preserve case information.
Why Update files are different
An Update needs to identify the existing record. If the Id is missing or malformed, ReadyRows blocks that row in Update mode so it is separated into the rejected export rather than silently passed through.
Upsert and External IDs
An Upsert may identify records using Salesforce Id or a custom External ID. ReadyRows can validate a standard Id column from the CSV, but it cannot know which custom field your org has configured as External ID without connecting to Salesforce.