yes
I think you mean "decibel", which is a unit for measuring the loudness or volume of sounds. A decimal is pertaining to ten or tenths, a fraction with a power of ten for its denominator, and a decimal point, which is a dot to show that the number following is a decimal.
300 ml is a measure of volume. A decimal is a number - pure and simple - neither mass, nor volume, neither length not time. So there is no decimal that is the same as 300 ml.
No.
no they are not always equivalent.
No.
Any number, negative or positive, can be expressed as a decimal number. So a negative number can always be expressed as a negative decimal.
Always
never
Volume = pi6210 = 1130.973 cubic inches to 3 decimal places
Volume = 4/3pi143 = 11494.04 cubic cm to 2 decimal places
Because a terminating decimal is a rational number that can also be expressed as a fraction
Volume = 4/3*pi*123 = 7238.229 cubic units to 3 decimal places