Based on this documentation, the REST API provides a way to submit a request for running a Flink job. The POST request must include the job configuration information as query parameters using the documented parameter names ("program-args", "entry-class", "parallelism", etc.)
Depending on the job parameters, the full URL for the POST request can reach a size that is over the maximum size (currently at 4096 bytes) of what is allowed by the configuration of Netty. To overcome this, it would be useful to allow users to provide the job configuration not only as query parameters but also as POST parameters.
For the most part, it is the "program-args" parameter that can make the URL grow in size based on the needs of the developer and the job. All other attributes should be pretty constant.