I have a database myDB in full recovery model. I took a full backup and restored it on the test server. Afterwards, I checked the size of T-log. The size of t-log was 15600 MB and used space out of total space was 351 MB. I decided to truncate those 351 MB and ran T-log backup. Unfortunately, the size does not change at all. It again showed 351 MB. Then I took other subsequent t-log backups. The size still 351. Why t-log backup does not truncate log file? I am not asking about shrinking, I just want to know why t-log does not truncate t-log.
Here are some details regarding t-log:
About the size of T-log: