What is the difference between these two situations?

1 External feature Dia 10 +/-0.1 with a Position of 0.1 LMC with respect to A, B, C

At LMC, 9.9-0.1 = 9.8
At MMC 10.1 = 10.1

2 External feature Dia 9.9 +/-0.1 with a Position of 0.1 MMC with respect to A, B, C

At MMC, 10+0.1 = 10.1
At LMC, 9.8 = 9.8

I am a GD&T learner and I do not have much exposure to GD&T stuff.

Could you please explain what is the difference between these two situations?

When should I use LMC or MMC, since both have the worst cases of 9.8 - 10.1