When importing an image sequence, Composite determines the resulting composition properties with respect to channels and bit depth. Some controls that let you override these default properties include:

• Output channels RGB, RGBA, and A.
• Bit depth either 16-bit or 32-bit regardless of the files original depth.

Your initial bit depth values can vary depending on the properties of the image files.

• From a jpeg file, the default proposed is 16-bit
• From a dpx 10-bit int file, the default is 16-bit
• From a dpx 16-bit int file, the default is 32-bit
• From an hdr 32-bit, the default is 32-bit

You can always create an RGB, RGBA or A Output Channel composition. Any missing channels are filled with black.