Although by default some of those properties are not displayed at first, I would not classify any of them as ‘new properties’.
The “clustered” property is to define which key in NAV will be used as the clustered index on SQL Server. To learn more about what exactly a clustered index is, you should search this forum and www.mibuso.com, there are hundreds of posts about it. Try also Books Online (or BOL for short), which should always be your first stop for information about SQL Server. Click the start button, all programs, SQL Server. For SQL 2000 BOL is right there, for SQL 2005 it’s under documentation and tutorials.
MaintainSQLIndex defines whether the NAV key is maintained on SQL Server. By unchecking that property, you will cause the system to drop the corresponding index on SQL Server.
MaintainSIFTIndex defines whether the SIFT buckets are maintained on SQL Server. On NAV 5.0 SP1 this is done by indexed views, and previous versions it is done by SIFT tables. Turn that property off, and the system will drop all SIFT objects on SQL Server.
In a nutshell, SIFTLevelsToMaintain determines which SIFT levels are maintained. When you create a key with 3 fields, and you add a sumindexfield to the key, and the MaintainSIFTIndex is turned on, by default the system will maintain 3 SIFT levels (one for field1 only, one for field1 and field2, and one for fields 1, 2 and 3). Going into the SIFTLevlesToMaintain dialog, you can see which levels are actually maintained, and you can specify which ones you want to be maintained.
SQLIndex is used to define the index on SQL Server that corresponds with the key in NAV. So if you have a key in NAV for document type and document number, you could set up a SQLIndex with the fields in reverse order, to make the index more selectable.
These are just short descriptions, search for all of them on the forums and you’ll find MANY posts about them.