Here's how Microsoft's support article expressed the matter:

If SP3 is installed, Microsoft advises against later uninstalling it, as there can be problems if the new features that were initially unlocked by SP3 get accessed. Microsoft's announcement also gave notice that a new Feature Pack for SQL Server 2014 SP3 is expected to arrive sometime in 'mid-November 2018.' A Feature Pack is a 'standalone package' that adds value to the server, according to Microsoft. IT pros may be cheered by getting full dumps (instead of mini-dumps) when 'replication agents encounter an unhandled exception.' Microsoft is also promising that SP3 will enable the 'proper handling of Service Broker' when there's an Availability Group failover as all open connections will now get closed with SP3. Microsoft also added two new DMVs for monitoring transaction log information. For instance, there's a new view to help administrators plan 'TempDB sizing' per database version storage requirements. There were other improvements, including Dynamic Management Views (DMV) additions. Database restore improvements for compressed '4K-sector volumes.'.Backup performance improvements for small to medium databases on large-memory machines.T-SQL syntax support for loading tables other than the 'default filegroup.'.Improved query handling based on CPU thresholds.Improved performance for 'change tracking cleanup.'.An improved distribution database cleanup procedure.It's a long list, but here's the short version: Microsoft's announcement described a number of improvements brought by SP3. SP3 contains all past SQL Server 2014 hotfixes, as well. SP3 is a 'cumulative update,' which means that it'll upgrade 'all editions and service levels of SQL Server 2014 to SQL Server 2014 SP3,' Microsoft's support article explained. The new service pack is arriving about two years after the release of SP2, per Microsoft's lifecycle page. It adds 'performance, scalability and diagnostics' improvements, according to Microsoft's announcement. SP3 brings new capabilities to Microsoft's relational database management system. Microsoft on Wednesday announced the release of SQL Server 2014 Service Pack 3 (SP3).

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