Q1: What does a card with "X in its text" refer to?
It refers to a card that contains the specified text or icon in its name, traits, effects, inherited effects, (Rule), digivolution requirements, DNA digivolution, DigiXros requirements, burst digivolve, App Fusion, or Link. For example, a card with Knightmon in its text would include cards with the name DarkKnightmon and cards with the text Knightmon in their effects.
Q2: If I use this card's Main effect to place a card under a Tamer with cards under it, in what order do I place the card?
The card is placed on the bottom of the cards under the Tamer.
Xros Up BT21-099 C Hunter [Main] You may place 1 Digimon card with <Save> in its text from your hand or trash under any of your Tamers. Then, 1 of your Digimon may digivolve into a Digimon card with <Save> in its text in the trash. --- [Security] You may play 1 card with <Save> in its text and a play cost of 5 or less from your hand or trash without paying the cost. Then, add this card to the hand.
Xros Up BT21-099 C Hunter [Main] You may place 1 Digimon card with <Save> in its text from your hand or trash under any of your Tamers. Then, 1 of your Digimon may digivolve into a Digimon card with <Save> in its text in the trash. --- [Security] You may play 1 card with <Save> in its text and a play cost of 5 or less from your hand or trash without paying the cost. Then, add this card to the hand.
Xros Up BT21-099 C Hunter [Main] You may place 1 Digimon card with <Save> from your hand or trash under any of your Tamers. Then, 1 of your Digimon may digivolve into a Digimon card with <Save> in the trash. ---