Hi there,
By default WordPress stores Transients Cache in Database. When a persistent object cache is available, it switches Transients from Database to the object cache.
Some plugins use Transients not in the proper ways, it stores too large data without expiration time. Without expirations, WordPress will place Transient in the “alloptions” variable. It will make persistent object caching solutions like Docket Cache can’t handle it.
The ideal “alloptions” size recommended by most hosting providers is not more than 1MB. Considering Docket Cache is file-based, we allow 3MB by default for each object. It can be changed by defining the DOCKET_CACHE_MAXSIZE constant.
So, “Retain Transients in Db” as the label suggested, will keep Transients in Database without switching it to a persistent object cache.
References:
Thanks.