ACR9000 Event Counters

What do the filter values in the subscription requests mean? Are they bitmasks? If so, which alarm does each bit correspond to?
"Filter Values" are bitmasks as shown in the help file for the "Event Counters". The "Bit Index" is used to calculate the "Controller Alarm Filter" value. The "Bit Index" is the binary weight for that particular parameter. To monitor "Stall Detect" as an Ethernet alarm, set the "Controller Alarm Filter" to "32" (2^5).

P36864 - P37096 Event Counter Parameters 

			(OS Version 1.18.13 & Up)

		Field                 Bit                            Object Number

		Description          Index       0      1      2      3      4      5      6      7  

	Master Move Started    0       36864  36896  36928  36960  36992  37024  37056  37088  

	Master Move Ended      1       36865  36897  36929  36961  36993  37025  37057  37089  

	Enable Input Lost      2       36866   N/A    N/A    N/A    N/A    N/A    N/A    N/A  

	Drive Faulted          3       36867  36899  36931  36963  36995  37027  37059  37091  

	Limit Hit              4       36868  36900  36932  36964  36996  37028  37060  37092  

	Stall Detect           5       36869  36901  36933  36965  36997  37029  37061  37093  

	Program Complete       6       36870  36902  36934  36966  36998  37030  37062  37094  

	Encoder Disconnect     7       36871  36903  36935  36967  36999  37031  37063  37095  

	Encoder Phase Error    8       36872  36904  36936  36968  37000  37032  37064  37096  

	CANopen Node Failure   9       36873  36905  36937  36969   N/A    N/A    N/A    N/A

The "Object Number" value across the top denotes which Master, Axis, Program, Encoder, CANopen node, etc. that that particular Event counter applies to. This table is a little unusual as it is much different than the rest of the bit and parameter tables.
NOTE: The ComACRserver does not support Ethernet Alarms
What do the different values for "Controller Alarm long" and "Object identifier" mean?
Once you receive the Ethernet alarm for a "controller alarm", then you will know which error occurred and which object number (object identifier seen in table above) had the problem, as well as the number of failures shown in the event counter. This is useful for intermittent problems as the event counter would show more than one failure on that object.

How does a User Alarm differ from a Controller Alarm? (I have generated Controller Alarms but never a User Alarm.)
The User Alarm is generated by setting bit 112 as stated in the manual. The bit has the legacy title of "Trigger AT Bus Interrupt" but it serves the same function here. Before setting bit 112, your AcroBASIC program running in the controller should write a value into the "User Alarm" parameter, P7140 (not documented.) This value is anything you want it to be. It could be a integer representation of a series of bits or a some other "error code."