The general rule is on page 4, under “One Round of the Game (Day or Night)”, item 2c:
• If at least one Core tile has been revealed, alternate dealing Elite (gold) and Regular Units. (Elite, Regular, Elite, etc.)
An exception is given for the Conquer and Hold scenario, page 19:
• Deal Regular (silver) Units only during the first half of the game. Deal both Regular and Elite Units during the second half (from Round 3 in a four player game, or from Round 4 in a three player game).