There is a good bit of information on this forum about the 3 Nuke ratings and what they do, but there is actually a 4th. The job title is Engineering Laboratory Technician (ELT). It is a branch off of the MM rating.
One of the main functions of ELT's is chemistry control in the reactor and steam plants. Anywhere there is water in the propulsion plant (reactor, steam generators, auxiliary support systems, makeup water systems), the ELT's will take samples of said water at the required frequencies and analyze it for various chemical parameters (and radioactivity if it is a reactor coolant sample) and adjust them as necessary to maintain them within the required specifications. You're basically a glorified pool boy/girl.
The other main function of ELT's is radiological controls (RADCON) on board the ship/sub. This part of the job involves performing routine radiation and contamination surveys throughout the propulsion plant and certain parts of the ship, covering work on any system where radioactive liquid is (or may be) present to prevent the spread of contamination outside of the designated areas, and responding to any casualties that may occur.
On submarines there are usually only 4 or 5 ELT's, including the Leading ELT. Carriers have a few more and, from what I understand, are sort of "unionized" where some ELT"s only do chemistry and some only do RADCON. Sub ELT's do everything.
To become an ELT, they say you have to volunteer, but I never did. I was "volun-told" to go to ELT school. There is an application process that will happen toward the end of Prototype. A good GPA throughout the pipeline is a good start to being accepted, but the real trick is to get to know the ELT's on your crew at Prototype as they have a good bit of influence on who gets picked up.
Since ELT's are MM's, they will qualify and stand all of the mechanical watches. On subs, most of the ELT's get stuck standing watch on the watch station where the steam generator analysis cabinet is and never really stand the other watches on a regular basis. I encourage anyone who becomes an ELT to buck that trend. I refused to let it happen to me because I liked being a mechanic too much and you still have to have the mechanic knowledge to be competitive on advancement exams.
Being an ELT is not necessarily better than being a regular Nuke MM, but it does give you another skill set that may make you more marketable to a potential employer. I got hired as a Chemistry Technician at a commercial nuke plant when I retired. I currently work as a Radiation Protection Supervisor for a company that is decommissioning a Cold War era Uranium processing facility. I have never really quit being an ELT since I left the Navy.
If you like chemistry or RADCON sounds interesting to you, I encourage you to give it a shot.