max-socket-threads - All the access to the Teiid (JDBC, ODBC, etc) is controlled by "transport" element in the configuration. Socket threads are configured for each transport. They handle NIO non-blocking IO operations as well as directly servicing any operation that can run without blocking. For longer running operations, the socket threads queue with work the query engine.
max_threads - The query engine has several settings that determine its thread utilization. max-threads sets the total number of threads available for query engine work (processing plans, transaction control operations, processing source queries, etc.). You should consider increasing the maximum threads on systems with a large number of available processors and/or when it's common to issue non-transactional queries with that issue a large number of concurrent source requests.
max-active-plans - Should always be smaller than max-threads, sets the number of the max-threads that should be used for user query processing. Increasing the max-active-plans should be considered for workloads with a high number of long running queries and/or systems with a large number of available processors. If memory issues arise from increasing the max-threads and the max-active-plans, then consider decreasing the processor/connector batch sizes to limit the base number of memory rows consumed by each plan.
thread-count-for-source-concurrency - Should always be smaller than max-threads, sets the number of concurrently executing source queries per user request. Setting this value to 1 forces serial execution of all source queries by the processing thread. The default value is computed based upon 2*max-threads/max-active-plans. Using the respective default values, this means that each user request would be allowed 6 concurrently executing source queries. If the default calculated value is not applicable to your workload, for example if you have queries that generate more concurrent long running source queries, you should adjust this value.