SP6 FAQ: http://www.microsoft.com/ntserver/support/faqs/sp6faq.asp#16

Because at the time of release United States restricted the export of strong
encryption software, SP6a was made available in a Standard Encryption
(56-bit) version for international languages and a English High Encryption
(128-bit) version for North America. The US lifted their export controls on
January 2000, and the English High Encryption (128-bit) version is now
available for download and installation worldwide.

Previous versions of Windows NT Service Packs used 40-bit Standard
Encryption. 56-bit encryption is more secure than 40-bit encryption, but not
subject to as many export restrictions as 128-bit.

SP6a is designed to upgrade systems currently using 40-bit encryption to
56-bit. SP6a can also upgrade Standard Encryption to High Encryption.
However, SP6a setup cannot downgrade High Encryption to Standard Encryption.
It's recommended that you not install High Encryption on a system that may
need to be exported.

If you are unsure which encryption level of Windows NT was originally
installed on a server, you can check by right clicking on Schannel.dll in
System32 and checking the file description.

To check encryption level:
Click Start, point to Programs, and then click Windows NT Explorer.
Click the Winnt folder and then click the System 32 folder.
Right click on Schannel.dll, click Properties, click version tab and then
view description.

If the description indicates "Export version", you have a 40-bit system. If
it indicates "U.S. domestic version," you have a 128-bit system.