one's knowledge on a particular subject

to have knowledge on a particular subject

not always, not necessarily

There is nothing better than ~ / ~ is the best / Nothing beats ~

to be limited to

be limited to, be restricted to / restrict

range, region / area, field, sphere / domain, territory

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend