When I play Sniper Training Mode in PUBG mobile sometimes I get "All Rifles" or "Bolt Action" written in bottom right of screen. So what is the difference between the two?
The message is indicating which pool of weapons are available for the round. Bolt Action refers to sniper rifles which do not have a magazine - the weapon must be reloaded after each shot (causing the scope view to end & potentially leading to you losing sight of your target as they scramble for cover). For more information on the type of rifle being modelled, please see this link: https://en.wikipedia.org/wiki/Bolt_action.
All Rifles expands this out to include sniper rifles with an ammunition magazine as well (allowing multiple shots without the need to reload).
Generally speaking, the bolt action rifles are more powerful than those with a magazine, helping to balance the inconvenience of needing to scope out and reload after each shot. Rifles with a magazine are less powerful but more forgiving, allowing for several follow up shots in quick succession.