When MediaInfo reports Bit rate, it'll report the bit rate the actual media stream specifies. So, if it says 1,500 and a constant bit rate was used, then this is the rate that was set by the encoder.
On the other hand, when a variable bit rate mode was used for encoding, the encoder cannot supply you with a predefined average. So, MediaInfo will report the measured average. It does that by analyzing the stream size and the duration (see File__Analyze_Streams_Finish.cpp in its source code), in a very generic fashion.
When it gives you a Maximum bit rate for VBR, the meaning is pretty evident. It also reports Maximum bit rate in the case of CBR streams. Unfortunately I haven't been able to find out what it means exactly—the source code is a little convoluted—but it depends on the specific media codecs or containers used.
Could be one of these:
- the result of a calculation, searching for a peak bit rate (very unlikely, haven't checked the full source code)
- a piece of information extracted from the MOOV atom of the MP4 container
- the MPEG-2 descriptor defined in ISO/IEC 13818-1 section 2.6.26:
The value indicates an upper bound of the bitrate, including transport overhead, that will be encountered in this program element or program
- the maximum bit rate supported by a certain container, e.g. in MXF
- the maximum bit rate supported by a certain codec profile / level combination
By the way: The MP4 (MPEG-4 Part 14) container has no restrictions on bit rates of the codecs used as far as I know. MP4 is not a video codec. If you're talking about "MP4" as in MPEG-4 Part 10 AVC video (h.264), then the maximum bit rate specified by the High 10 profile at level 5.2 is 720,000 kBit/s (reference).