A. Connected Lookup Receives input values directly from the pipeline. We can use a dynamic or static cache Supports user-defined default values
Unconnected Lookup Receives input values from the result of a :LKP expression in another transformation. We can use a static cache Does not support user-defined default values
When you compared both basically connected lookup will return more values and unconnected returns one value. conn lookup is in the same pipeline of source and it will accept dynamic caching. Unconn lookup don't have that faclity but in some special cases we can use Unconnected. if output of one lookup is going as input of another lookup this
unconnected lookups are favourable Moreover if the mapping requires multiple lookup using same lookup conditions then its better to use unconnected lookup & call the lookup wherever required.This also prevents the mapping from being complex.