What really Sekha means? i'm still a bit confused, 
i know that "sekha" means literally "a learner; in course of perfection"
but in some article "sekha" means "a pupil or one under training in a religious doctrine."

what i really want to know is if whether the word "sekha" is used for religious terms only, or it can be used in many terms, for ex. student at school or disciple from someone else to reach knowledge or something like that?
In Saṅgītisutta three types of persons in the world are mentioned: sekkha (learner), asekkha (adept) and nevasekkha nāsekkha (ordinary person or puthujjana)


  Tayo puggalā—sekkho puggalo, asekkho puggalo,
  nevasekkho nāsekkho puggalo
  Saṅgītisutta
  Three kinds of persons, to wit, the learner, the adept, he
  who is neither.
  Saṅgītisutta [en]


A sekha is someone who trains himself in higher virtues, training the mind to a higher degree and training for higher wisdom or basically someone who's training himself in the eightfold path. There are 7 types of sekhas (1) the path to stream-entry; (2) the fruition of stream-entry;
(3) the path to once-returning; (4) the fruition of once-returning;
(5) the path to non-returning; (6) the fruition of non-returning;
(7) the path to arahantship


  Sekho, sekho’ti, bhante, vuccati. Kittāvatā nu kho, bhante, sekho
  hotī”ti? “Sikkhatīti kho, bhikkhu, tasmā sekhoti vuccati.
  Kiñca sikkhati? Adhisīlampi sikkhati, adhicittampi sikkhati,
  adhipaññampi sikkhati. Sikkhatīti kho, bhikkhu, tasmā sekhoti
  vuccatīti.
  Sekkhasutta
  
  Venerable sir, it is said a trainee. How is one a trainee?
  Bhikkhu, one trains, therefore he is called a trainee.
  In what does he train? He trains in higher virtues,
  training the mind to a higher degree and training for higher wisdom.
  Sekkhasutta [en]


An asekha is someone who's gone beyond the training or basically an arahant.


  Asekho asekho’ti, bhante, vuccati. Kittāvatā bhante, bhikkhu asekho hotī”ti? “Idha, bhikkhu, bhikkhu asekhāya sammādiṭṭhiyā samannāgato hoti, asekhena sam­māsaṅkap­pena samannāgato hoti, ... Evaṃ kho, bhikkhu, bhikkhu asekho hotī”ti
  Paṭha­ma­a­sekha­sutta
  
  Venerable sir, it is said, `gone beyond the training' How is the bhikkhu gone beyond the training?
  Here, bhikkhus, the bhikkhu is endowed with perfect right view,
  perfect right thoughts, perfect right speech, perfect right activity,
  perfect right livelihood, perfect right endeavour, perfect right
  mindfulness, perfect right concentration perfect right knowledge and
  perfect right release gone beyond the training. Such a one is gone
  beyond the training.
  Paṭha­ma­a­sekha­sutta [en]