what is the purpose of a side deck in yugioh ARTMAGE PROFILE What does side deck mean?

In addition to these free spells, each Commander 2020 deck contains additional new commanders that can be used in place of the main general

while this card is face-up on the field, you can Normal Summon Level 5 or higher Dinosaur monsters without Tributing

its just that I mostly have experience with lower-value bulk hits from those sets

C Common Kashtira Akstra X SPELL Quick-Play If a face-up "Kashtira" Xyz Monster(s) you control is destroyed by battle or card effect: Banish 1 "Visas Starfrost" from your hand, your Deck, or your Monster Zone, and if you do, Special Summon 1 "Vicious Astraloud" from your Extra Deck, ignoring its Summoning conditions