What is the Difference between synchronizer and lockup latch?

What is the Difference between synchronizer and lockup latch?

A Synchronizer handles situations where data needs to move from faster clock domain to slower clock domain or vice-versa,whereas lockup latch doesn't allow data to cross between two clock domains.

Lockup latch is used to add a half cycle to avoid setup/hold issues between two flops.

Lock-up latches are used to allow scan chains to cross the clock domains. They mitigate the skew between two clock domains to ensure data is shifted reliably on the scan chain.