Minecraft wiki entry for data_types, associated text:  Uniform Int:[needs testing] This structure represents a randomly generated integer between base and base + spread, inclusive, where each number in the range has equal probability. spread must always be greater than or equal to 0. Additionally, different configuration settings often place extra restrictions on the range of these values; these restrictions are expressed as minBase, maxBase, and maxSpread. All of these restrictions are inclusive ranges. If these restrictions are not stated, then any value of base and any non-negative value of spread is allowed. Alternatively, the compound tag can be substituted for an int tag, in which case the provided value is used as base and spread is set to 0, effectively making this the same as a constant integer. In this case, minBase and maxBase still apply to this value if they are provided.
 base: The lowest number which can be generated.
 spread: The size of the inclusive range to generate numbers within.