[2.9] Ambgious way how filters works

[SP8EBC]

I have configuration like that

<source>
        source          $mycall
        relay-type      directonly      # default mode is "digipeated"
        viscous-delay   2               # no viscous delay for RF->RF digipeating
        ratelimit       10 20   # default: average 60 packets/minute,
                                #          burst max 120 packets/minute
        <wide>
                maxreq 2
                maxdone 2
                keys WM,SP
        </wide>

        <trace>
                maxreq 2
                maxdone 2
                keys WIDE
        </trace>
#    ##  filter         a/la/lo/la/lo  # service area filter
        filter         -b/SR*        # always block these
        filter          t/*
</source>

The problem is that if I leave filter -b/SR* alone nothing got digipeated, even stations which doesn't match with the filter definition. To have it working as I want I need to add one line extra filter t/*

The same problem occurs in TX-igate configuration

<source>                
        # APRSIS source adds a TX-IGATE behaviour
        source          APRSIS
        relay-type      third-party  # Must define this for APRSIS source!
        viscous-delay   5       # Recommendation: 5 seconds delay to give
                                # RF delivery time make itself known.
        ratelimit       60 120      # default: average 60 packets/minute,
                              #          burst max 120 packets/minute
        filter          -b/OK8OUF
        filter          -b/cinopqwt
        filter          t/ms
        filter          m/25  # service area filter
</source>
#

If I will leave only filter -b/cinopqwt the Igate will transmit nothing. To have messages and status transmitted Internet -> RF I need to explicitly define the next additive filter. The official user manual doesn't explain exactly how this work.