Apex to back, in a bevel gear or hypoid gear, is the distance in the direction of the axis from the apex of the pitch cone to a locating surface at the back of the blank.
Salary DOE means that the salary depends on experience of the candidate. A similar term is DOQ which means salary depends on qualification. Be careful with DOE job offers with small companies where they only can fill one position. Every position ...
Offering salary DOE means that the employer has a wide range of experience requirements that makes me wonder if the job mission, goals and therefor experience requirements are well defined. That is clearly a subject you want to address in the interview.