From the reference for PARALLEL_MIN_PERCENT:
PARALLEL_MIN_PERCENT lets you specify the minimum percentage of the requested number of parallel execution processes required for parallel execution. Setting this parameter ensures that parallel operations will not execute unless adequate resources are available. The default value of 0 means that no minimum percentage of processes has been set.
Consider the following settings:
PARALLEL_MIN_PERCENT = 50
PARALLEL_MIN_SERVERS = 5
PARALLEL_MAX_SERVERS = 10
If 8 of the 10 parallel execution processes are busy, only 2 processes are available. If you then request a query with a degree of parallelism of 8, the minimum 50% will not be met.
[PARALLEL_MIN_SERVERS] Specifies the minimum percentage of requested parallel execution processes required for parallel execution. With the default value of 0, a parallel statement executes serial if no parallel server processes are available.
What happens when the minimum 50% of servers is not available for the query when it runs? Is the query is queued, does it execute serially, or does it throw an error such as insufficient parallel query slaves available?